First I agree with the two posts up there.
what you did wrong:
1. you orally agreed to work 6 hours with 4-hour pay.
2. sending email is putting a written record on this matter and may put her into a very difficult position with HR.
what you can do:talk with her
1. explain that you did not mean to change working hours and thus, did not mean to get more pay and certainly did not mean to cause trouble for her.
2. you did your best to help her with her task by putting more hours into work without more pay. But from now on because of the classes, you can only work for 4 hours.
3. look for another job.
