A motorcycle suddenly and very loudly crashes through the wall (the wall outside, near the zombies), followed by a female voice shouting "AH, FUCK!".

several dozen bodies turn and start converging on the noise

Elias has a blank look Christine thinks he's dissociated

She grabs his shoulder then gestures for them to try and sneak past the horde well it is distracted

poor woman god have mercy

With incredible speed, Amanda makes an acrobatic stand, draws her pistol, and starts to back away while quickly scanning the area.

It's a lot of monsters. Hard to count how much. The situation doesn't seem completely impossible, but she doesn't have enough ammo for all of them, and will need a lot of luck to survive.
Are there any escape routes? Are any of the zombies coming at her from the back? Is there anything around her she can use to either escape or fight?

She maneuvers around the broken motorcycle. It can be used as a sort of very small barrier. Or to stand on if she needs a bit more height. A big enough pile of bodies will also create a useful height. Is her balance good enou...yes it is.
She is not strong enough to use the remains of the motorcycle as a weapon. Or even throw it, probably. But she can just lift it for a second, maybe push. Would that be useful? Probably not very much.

The creatures appeared to be most densely clustered near a broken down classroom door 

With less in other parts of the corridor

There are also several climbing their way through the whole the motorbike left in the wall 

She can also see two teenagers attempt to sneak thro said classroom door as the bulk of the horde move towards her

"Your'e alive?" she mutters, slightly surprised.

But she doesn't have time to think about it.
Hands together on the pistol. Legs in the right position. Eyes and shoulders towards the target.

Bam! Bam! Bam!
She hits each zombie in the forehead, killing them instantly. Well, she missed one of them. And she does it slower than she could if she didn't prioritize accuracy.
But still, in 7 seconds 9 of them are on the ground.

The rest of them respond to the noise and the smell of blood in the air by  surging forward quicker and more aggressively

He stops transfixed by the woman seemingly effortless speed

It's not really effortless, but it is incredible. Very near the possible human maximum.
(She is also beautiful, if only under a specific archetype of beauty, especially if you also like biker leather fashion).

She makes two more shots. Reloads the magazine, very fast, though not as fast as she would want.
Runs several steps to the back (from her relative position) of the corridor (the speed of her steps is high, but unlike her movement and coordination not much faster than you would expect from an average person), as the zombies are too close.
She shoots 5 of the zombies closest to her on the back, can't let herself be sorrounded.
The other in the back won't reach her in the next 10-20 seconds, at least. The hole in the wall is not that big, and there weren't much zombies outside, certainly not close to it. So no significant amount will come through in the next minute or two. The main horde is the problem.

She turns back to the main horde. Shoots, with maximal precision, the last several bullets. Targeting the zombies slightly farther from her, not the closest ones. So the closest zombies reach her slightly faster, in several moments, but not many of them at once, only one or two. At first.
She holsters the gun, and waits, in a pose with arms stretched and head leaning forward, and left leg bent backwards, for the nearest zombie to be in reach.

A zombie lynches towards her

With blinding speed, she:
Grabs the zombie's hand;
Pulls the zombie towards her and sideways, while moving her head and upper body backwards;
Moves her legs to half-trip it;
And executes a perfect judo throw, throwing the full weight of the zombie back into the horde – with surprising ease – dropping it and two others to the ground.

Zombies are not good at adapting to changing situations; she can keep doing it again.
Zombies do not feel pain, and need no time to catch their breath after being forcefully thrown on the ground. But they are slow, clumsy and stupid, so standing back will take them some time, especially when there are several on top of each other, in the middle of a horde that will not be polite enough to stop trying to reach their goal (Amanda) as fast as possible.
If she could drop 5-6 zombies with every throw, – as she hoped  – the numbers would be in her favor.
But even her enhanced strength is not (yet?) enough to achieve that.

She draws a knife. She's not as skilled with melee weapons as she with firearms and unarmed subdual, but that will have to do.
She maneuvers back to the broken motorcycle, parrying zombie lunges with a knife (if they try to bite rather than use their hands, it might even result in killing them). Drags a broken locker, and anything else she can find lying on the ground, creating a very improvised (she is quick, but only has seconds) barricade.