Ahh the rights of passage! I think I graduated from high school the first year they ever had a senior lock-in party at a high school. That may not be accurate, but I do not recall these sleep deprived overnights taking place prior to that.

At our senior party we could have our photo taken with a giant boa constrictor and eat gummy bears and drink orange soda to the point of combustion.
In the morning, we were released from the gym with a beer mug with our graduation year and school name, and a tshirt with each graduate’s signature screened on the back.

Today, I doubt many high schools allow the booster club to give each student a beer mug. We do however get a fair number of tshirt orders for senior parties. For a graduating class of 150 kids, you can get a white high quality shirt printed with a one color imprint on the front for about $4.50/each, add a back imprint and figure on about $5.75 each.
http://www.imprintitems.com/T-shirts.htm

Another popular end of the year gift is a laundry bag.
http://www.imprintitems.com/custom/cat/topic-laundry-bags-sacks

The idea on this giveaway is that the graduates will move out, go to college, etc. and may need to carry dirty laundry to the laundrymat or laundry room on campus. The reality is, though many graduates will move out of their parents’ homes, most will use the laundry bags to transport dirty laundry back to mom and dad’s house for a cheap wash. Regardless, the laundry bag is a nice promotional giveaway to graduating high school students and incoming college students as well.