Should I write a Thank you card?

You should definitely write your teachers thank you cards!! I'm in 11th grade and I've written long letters to a few special teachers as well as shorter ones to other teachers over the years. It's so thoughtful of you to care enough to thank your teachers and don't feel like you're sucking up to them, because you aren't! You are appreciating their hard work. (Especially if you give it to them at the end of the year, it's not like you're bribing them to change your grade or something.)

It would be nice to give them to all of your teachers, but is certainly not a "requirement." You could always write a general thank you to all of your teachers and go more specific for the one's you are close with.

If you're worried about what to say, on your way out of your last class just hand it to your teacher and say "thank you!"

For a teacher you are closer with, you probably are going to say goodbye in a more meaningful way, so perhaps say something like "Thank you so much, I've loved being in your class this year!"

Or if you really don't think you can do it in person, leave the card on the teacher's desk when they aren't around and they'll find it.

Well that's a very good idea..many people do not take the time out to thank their teachers. Even the Bible says: “A word at its right time is O how good!”—Proverbs 15:23.
And if you are doing it from the heart, it shouldn't matter what people may think. And when you are writing to each of your teachers, be sure to personalize it and when you write to the others, generalize your card. In it you could possibly add:
" I just want to take the time to thank you for teaching us. I know how hard it may be to teach children these days so I just want to say I appreciate your efforts in teaching me, as well as my fellow students. Please know that it wasn't done in vain; I will treasure your teachings and strive to use them in my life. Keep up the good work and I hope you have a good life/spring break/summer (whatever the occasion allows)."
Make each letter personal to the teachers you knew on a day to day basis. You can even include the scripture here at Luke 6:40 which says, "A pupil is not above his teacher, but everyone that is perfectly instructed will be like his teacher.'
Hope this helps! And that's so sweet of you to do this - keep up the good work!