Do I have to pay 19.99 a month for a full year, or can I just pay 19.99 for the few months I need adobe for school
The $19.99 student deal is only available with a 12 month plan. Total cost $239.88.
The only monthly plan is the Individual (non-Student) Full Cloud @ $75/month. After 3 and a bit months, you're better off with the Student 12 month plan.