Good question, and kudos for asking. It's been a while, but here are a couple big differences:
1) What you had in a year will be compressed into one semester.
2) There will be 3 or 4 tests, and most likely not a lot of homework to make your grade, so your exams matter more. If you have a cumulative final, it will be a year's worth of HS material that you'll have to review in a week or so. (So, IOW, learn that stuff cold throughout the semester so you don't have to teach yourself at the end.)
3) Your tests for all classes will come in blocks, because every professor splits their semester up roughly the same way. In my HS the teachers collaborated to put some distance between the tests.
4) You'll have a lab that takes up a lot more time than HS labs. I was lucky, I went to an HS where labs were hard, but the expectations for this 1 hour class alone would've kicked a few of my old friends' butts.
This is possible because you will have 4 classes instead of 8. You will most likely dive a little deeper into some concepts, and memorize that one little detail that your HS professors told you not to worry about.
Keep an eye on Facebook, and compare the number of freshman and sophomore bio/chem/phys majors vs. juniors and seniors. That'll make you feel better later on if you can keep it up.
Lastly, here's this fun article:
Math, Science Popular Until Students Realize They're Hard.
