A simple yet effective way to design your senior T-shirt is to get all the students in class to write their names on it. Mark 2011, or the year you will graduate, prominently in the middle of the front or back of the shirt. If you have teachers you like, save a spot on the shirt for their signatures too. Because everyone's T-shirt will look different, you should get same-colored T-shirts for everybody. If you want to get your names printed on the T-Shirts, there are places that will do that for you too.
Even if gradation means you won't be seeing your friends everyday anymore, it also means that you are now done with school, and that is a very good reason to be happy. Ask your classmates to come up with a funny catch phrase to put on all your shirts and then vote for the best one. If there is an internal joke or anecdote you share in class, you should try to make a catch phrase out of that. Don't forget to put the year 2011 in a noticeable spot.
Your senior class T-shirt can also be all about the year you will graduate. Put a big 2011 on the front end of the shirt. If your school colors are nice looking, use them to write the number. Alternatively, you can make a time line of your school years. Write the first year you started school and the year 2011 when you finish, in the "from-to" kind of way. You can also draw a tombstone on the shirt with the same time line and the words "R.I.P. High-school/College."
What better way of having your classmates remember you than by writing what you think of them on a T-Shirt. Encourage everybody in class to think about what they would write on other people's shirts. They can write whatever they want, as long as it will mean something to them and the person wearing the shirt. Remind your classmates to keep it short, so that everybody will get a space on the T-shirt. Buy blank T-shirts for everybody and some fabric markers. The T-shirts will differ from one another, but will be extremely personalized. The year 2011, in this case, can go on the sleeve.