I think the reason the $125 is "going to make the difference" is because he needs it in full by the 29th of August to get into school this year.

You're probably right. OTOH, I think that the OP could think about it in time, and not wait for August 25th to jump up and decide to go back to school.

It would seem likely a scammer would seek a larger amount.

I'd say the opposite. A small amount is more attractive to potential donors, and your post just proves it. Moreover, there is no control over how many persons are going to help the OP - 10 times $125 seem a large amount with this respect. Even if there would be a way for the donors to coordinate, if you were a scammer would you prefer to ask for $1000 and receive nothing, or ask for $125 and receive $125 because "it's not that large"?

The nature of this request makes it seem legitimate - a relatively small amount needed by a swift-approaching deadline.

This is the root of practically all scams. I have tons of e-mails saying that there is an unclaimed bunch of money waiting for me - but fast! And they would probably tell me that I'd pay a little amount of money for legal expenses (I've never gone beyond simply deleting these emails, so that's guessing).