What's a good solution for a couple extra hours on an 11" MacBook Air?
I am able to actively use my MacBook Air for around 7-8 hours. It is just a basic advice but it works fine for me:


Lower the screen brightness - it's relatively easy to control brightness on MacBook (F1 or F2).
Lower the backlit of the keyboard or, as your son is using his laptop in school during the day, it even makes sense to fully turn off the backlit (F5 or F6).
When the battery is getting to 25-30% I would quit all the cloud based apps like DropBox, OneDrive or iCloud. In Dropbox you can just pause syncing, which is a really handy feature.
Quit all the unnecessary apps. For example if in the class kids had a need to use MS Word and in the next class they don't need to use it, then show your son how to quit programs what he's not using.


I'm not sure if this will give MacBook Air battery two extra hours of life, but I would say from an hour to hour and a half.

One more thing would be somewhat reasonable usage of the battery. Meaning not leaving laptop to charge for hours when the battery is full and vice versa draining the battery all the way to 0% and then start charging it. Li-ion batteries don't like to be fully drained. Charging the battery with the mentioned above mindset could help it live longer.