Here's the deal. You can't upgrade the Air. Whatever you get now, you get forever.
You can upgrade the Pro. So you can get more RAM and a larger HD later on if you want it.
However, the Air comes with SSD, which is very desirable--and certainly faster; it is lighter than a Pro. And if you get the 11", then it's really lighter, will give you a lot more room in your backpack, and fit on very small desks in just about any classroom. The Pro is not so portable nor fits so well on desks.
My recommendation? First, go to a store and try out both. It's always important to get the laptop you fall in love with, as that's the one you'll want to work on. You can get around having a limited HD, but you can't get around working on a laptop you don't like. My suspicion is that you should probably the 11" Air which might give you extra $$ so you can upgrade to a 256gb HD (you'll have to order it online) and 8gb RAM. 128gb probably won't be enough for 4-5 years of school, because you never know what program or game or whatever you're going to want or need in those 4-5 years that might max out the HD. Better to get 256gb which tends to be enough for most people over four years, with room to spare. Especially given that people now keep a lot of information on their phones, clouds, etc.
Students haul around a lot of weight, and you will feel the extra weight of that Pro, especially as compared to an 11" Air. Also, you will be glad to have a small machine for your desk, giving you room for other stuff, and you will be glad to have a laptop that doesn't take up a lot of room in your bag or backpack, one you can slip in and out with one hand. The Air can go with you from class to class for the next four years without making you feel weighed down. So 11" Air would be my recommendation. But please don't take anyone's recommendation before going to a store and trying out both computers. You can only know which one is right for you if you do a little hands-on.