If you truly feel you cannot balance the internship and school you need to communicate that to them honestly. It can go good or bad depending on the style of the person you answer to over there.
On one hand it shows you are dedicated to your schooling, are cognizant of the negative effect(s) it could have for your future if your schooling failed and are learning important lessons like not overextending yourself and balancing commitments - which you will need to learn when you are in the work force. You can ask to shorten the hours and extend the internship, or they may just end it right there since you have already done 13 weeks, it is not like you just started and then got cold feet. Communicate the fact you are still growing, and would very much like to continue in some capacity and that it doesn't blacklist you or look bad.
On the other hand, it highlights the lack of critical thinking, forward planning and over-committing, which honestly should be expected - you're in school still, not experienced with life in the grand scheme of things. So if that manager happens to be harder on the approach they can get you blacklisted or complain to the school if that is how your got the internship.
I wouldn't worry about the bad results - but it is worth mentioning. Honesty is key and I would focus on school, who cares if you burn a bridge on the way to a degree, internships are nothing like the real thing anyway so don't have any qualms no what happens or what you choose. Either way you do not want to overcommit and burn yourself out.