Since I just graduated, I'll share my thoughts to help the new senior class. :]
~The first semester, honestly, will probably suck balls for you. You'll have to deal with college applications, on top of difficult classes, extracurriculars, and maybe even a job. A lot of my friends were feeling extra stressed about if their cumulative grades were good enough for their desired schools. Just try to find something fun to do to get your mind off it. Stress isn't something anyone wants, so if you can manage your time, you'll have an amazing time being a senior. :]
~With only a year left before you never see most of these people again, don't be afraid to take chances with that girl/guy. There's not much time left, so do it before you regret it later! :]
~In all honesty, grades won't matter after the first semester. As long as you don't fail most of your classes, colleges will ignore grade drops in the second semester. Unless aiming for valedictorian, work hardest first semester so you can relax and enjoy the remainder of your senior year.
~Lastly, don't place TOO much meaning into anything. If prom goes bad, or if your senior experince goes awry, don't sweat. This will NOT be the best time of your life. There will be better things ahead. Just have as much fun as you can, while you can. :]
Hope y'all have a great time!