Every Freshman and their parents are spending money on books, locker fees, uniforms and yes the official school shirt. Homecoming - Spend time doing a special Homecoming design and sell these shirts as a fund raiser or at the actual event. Class name shirts are still popular with graduating classes. Although the size of order many be large, the profits can be low. You will need to be ready to go and actually print overnight to make money on these opportunities.
Timing is everything here.
Local stores may want to carry the high school shirts just like they do college shirts. Spring Break - This is popular with colleges and it is spreading to the high schools. Create a hot Spring Break design and sell it through the student council or as a fund raiser. Although the t-shirt is the most popular, it can be the least profitable. These can be purchased from outside sources on a wholesale basis.