Can i do it?

I'm 17 years old, breastfeeding my 2.5 month old, and still in high school. I only have class for 4 hours so i decided to just have my daughter have formula at daycare and i don't pump while i'm at school, i feed her before and after. Since i go close to 5 hours without feeding/pumping, my supply went down and we do about half formula half breastfeeding. when school ends I would like to try exclusively breastfeeding again for the summer, is it possible for me to get my supply high enough to stop using formula? If so, what can i do?