Well, some of the modeling agency offers were pretty glitzy. But yeah, this is nice.

"Tell them I want a job if you don't."

"I thought you wanted to work for Paladins."

"They funnel people to the Paladins!"

"They funnel girls." Anyway, thus reminded of the options, she checks out the website.

The website design has the simple understated elegance of an organization that knows it's cool. It contains much more detailed information about the internship program and the various activities of the Pax Corps, as well as a list of partner groups.

Apparently, if someone doesn't seem a good fit for the Pax Corps but is interested in one or more partner groups, they also offer a variety of appropriate basic training and a recommendation, in exchange for a negotiable medium-term income sharing agreement if this process results in employment with a partner group. (The Paladins are one of the groups who routinely cover the income sharing costs for someone they hire this way.) 

(It also turns out that they do have a similar funnel process for mundane professionals, but the corresponding programs have steeper requirements and lower acceptance rates. The Pax Corps has invested a great deal of effort into finding promising applications for magical talent and is somewhat specialized therein.) 

Is there anything already on there about getting underage girls opportunities to use their powers in a medical setting or will she have to send a freeform letter?

The Pax Corps does not discriminate based on age. There are a couple of entries on the application form that concern U.S. standards for parent or guardian consent and how it is confirmed. (No particular answer is required for the application to be submitted, but a lack of consent would be flagged for review by a certain subdivision.)

Medicine is indeed one of the default interest options. 

Then she will get pro forma agreement from Renée and send in her application. She has the power to stop things and is interested in tests to discover if "things" include, like, blood clots.

A few days later, she receives an email inviting her to schedule two days of preliminary testing within the next few weeks, potentially but not necessarily consecutive. The first day of tests begins with plants (e.g. "can you stop this cutting from leaking sap?") and a handful of increasingly complex animals. For the second day, they propose to bring in a magical girl with biology powers for tests that are hard to perform or validate with medical equipment alone. (In the U.S., a typical test subject for this second day is a mouse in a temporary magically-induced coma, restored to full health before reawakening.) 

There's also a generic packet of information on the family of "slow/stop/stasis" powers, including statistics on where and how they've been employed in the Pax Corps (which has excellent record-keeping) and partner groups (which vary) over the decades. It's fairly sparse, as such packets go; this is not a common power. 

Sounds good. Is it time-consuming enough that she needs to take time off school?

The tests each take most of a day, but can be scheduled for a weekend. If necessary, they can provide a note for her school.

She'll take weekend scheduling.

The testing is conducted at a medical research facility in downtown Phoenix. A local staff member walks her through the process while others busy themselves with preparations and notetaking. As in the description, it's mostly plants and invertebrates at first. They test whether she can safely staunch bleeding, put a whole creature in stasis, affect specific organs she can or can't see, steady a broken limb, affect specific groups or types of organism (one species of ant in this group? One species of bacteria on this plate?) and various other permutations. They also test whether or to what extent things she stops are affected by outside forces, and whether she can alter this with concentration. 

She seems to only work on things that are at least one of "solid objects" or "complete and contiguous", so she cannot stop bleeding - blood's a liquid and you don't want all of it to stop. She can hold a limb still, that's solid enough. She can hold a creature in midair if she wants but it'll still be, like, experiencing the passage of time. She can only affect one target at a time, not multiple ants or bacteria. She can hold things against a lot of outside force though. She doesn't need to be concentrating except insofar as she needs to stay in range and not switch targets; she can stop a pillow in midair and sit on it and get lost in a book.

Can she hold a limb still relative to the body it's attached to, thereby letting them move both without worsening an injury? 

Yes but it has absolutely no give so it's not actually going to be that comfortable for them to move much.

Noted. They run some size and precision tests. Does she need to be able to see her target? Does a video camera suffice? The pillow trick implies she doesn't need to be looking directly at it to maintain the effect? Can she get something microscopic?

She does not need to be looking at it but she needs to know where it is. A camera will do if she's familiar with the place it's pointed at and its location relative to where she is. She does not need to look at it to maintain the effect. She can get something microscopic if she has a microscope.

There's a break for lunch during which the flurry of activity dies down a bit. "Thanks for your patience!" remarks Clara, the tech who's been walking her through the process. "We have some more tests scheduled, but there's also room to get creative in the afternoon as long as it's not dangerous. Anything you're particularly excited to try?" 

"I'd been thinking of, like I said on my form, blood clots - strokes and stuff - but it could be completely contraindicated. I am curious how big a thing I can stop, I've been reluctant to try it on moving cars."

"I'm not sure we can test bigness here. I guess we could do, like, a quick check in the parking lot before you leave, going slowly. Strokes are in tomorrow's list, but that one seemed a bit odd to me. What would you be stopping, exactly?"

"The blood clot? I may be operating on an incorrect model of how strokes work."

"I would naively expect that just freezes the clot in place, which is more likely to harm than help. If you can somehow stop the process of clotting itself, that's another story. Have you done anything that abstract before?"

"No, I haven't. I think I was imagining that the clot proceeds through the blood vessel and gets to more troublesome places in so doing and I'd be buying time for it to get removed or broken down with blood thinners or something but now that I'm saying this I'm realizing that I may have based my information on The Wonderful Story of Henry Sugar."