I can tell you from my personal experience that for me when I stopped the taking my ritalin I went through withdrawl. Neasua, dry heaves, sweats it was like goining through detox. So you may want to keep her on it for the summer just lower the dosage so that she does not have to go through that.
I also know that when I am not on my RX my behavior is more impulsive. I have been on the ritalin since I was 8yrs old without any breaks until I was 28. That is when I decided that I did not need meds. I was off of the meds for almost a year before my husband threated to leave if I did not go back on the meds because my behavior was so unattentive and impulsive.
I hope my experiences have helped you.