You can purchase basic Magic Bands from the Disney Store's website. Also make sure you set up a My Disney Experience account. Once you do those two thing, you can link the Magic Band to your account.
However, you also need to link the park tickets to your account, in addition to the Magic Band. To do this, you'll need the physical ticket, or at least the confirmation number for your specific ticket. I know when I went with school, we didn't actually get the ticket until the first day we visited the parks. If that's the case you could buy the Magic Band and link it to your account so you have everything ready to go. Then, when you get your park ticket, you can link it to your account via phone.
I have no idea about Memory Maker, sorry!
EDIT: I should also say - it probably won't be cheaper to buy it ahead of time. I think the basic magic bands are same price online vs. in the parks. In fact, you might be paying more money if you order it ahead of time because of shipping.
EDIT 2: If you are staying in a Disney resort, you will probably get a magic band anyway, although I'm not sure if it's different for school groups. I would check with the teacher before you buy anything!
