Hi Louise,
Here is the initial design idea that I cooked up for your seniors. I am pushing 28, but I still remember my final year of what I like to call the AwesomeYears™ ;)
Anyway, here is my first design in two colors (#5 & #6), just in case the girls would want their own color. However, I tried to incorporate elements that would appeal to both girls and boys, without it leaning too far towards either direction. I think overall, it would strengthen the effect of the shirts if both sexes wore the school colors, but that is totally up to them, of course!
I will see what else I can come up with, but feel free to comment on these in the meantime.
Thanks!