great post michiganfan those should be your priorities at your age, especially if you are a michigan student. don't completely blow off school because you'll want that diploma but you'll never again have the opportunity to revel in nightlife, hit chicks and drugs like you do now. make the most of it.
that being said, someone recommended london. scratch that off your list right now based on your priorities.
someone said plan a few extra weeks for traveling. that's great advice. whatever city you choose, there will be others you'll want to visit.
italy and spain are great places where the chicks are very hot. i would recommend barcelona if you decide on one of those countries.
i'd probably say no to amsterdam. obviously the drugs and nightlife are there but it's not as strong in the hot chicks department. plus, i wouldn't recommend amsterdam for an entire semester. it's definitely a great place to visit but i think you're better off staying in another city and taking a trip or two to amsterdam for a few days.
greece is outstanding. it has all you need (understanding that chicks, nightlife and drugs/alcohol are all you need at your age). greek women aren't anything special but if you are spending your time on the islands, you'll get hot chicks from all over. that being said, i don't know what study abroad programs would be on the islands. i would not go to athens. it's too big and nowhere near other european cities in the hit chicks department. plus greece is pretty far out of the way and would be difficult to get to other cities in europe. but if you can find some way to spend some time on islands like mykonos and ios, it's something to look into.
one thing i'd suggest is to look into the scandinavian countries. there are so many hot chicks up there, you won't believe it. on my first trip to europe, i had no intention of going to any scandinavian country and ended up spending half my time there.
prague is good. the chicks aren't as hot and prevalent as spain, italy or scandinavia, but it's a big party city and a nice place.
based on my experience, which is limited of course, i would say barcelona. great city and a pretty good location to get to other places. you can't go wrong with italy, but i'd probably stay in a northern city like florence. get to prague, budapest and scandinavia if you can.
good luck.