Three basic ways of greater profits!
1) Find more customers
2) Offer another product or service
3) Cut your overheads
No1 - you could obviously try to sell to more schools this would be the obvious route.
Why not put some offers in aswell, ie, if they buy 3 tshirts they get one for FREE , or if they introduce another customer who buys something from you they get 50% off their next purchase.
Try to get people to spend more and get your average sale price higher!
Try to put some offers in to get people to introduce you to new people, id bet half the people in your school live in the same neighbourhood as people from different schools - Find out how to reach them
2) Offer another product - I think the main one here for the Young crew surely must be baseball caps, if you could start customising these then you'd be bound to make a few quid
brainstorm what other things you could offer:
How about special occasions: valentines day, mothers day, fathers day, birthdays ect. I am sure there would be some ideas in their for you for some personised bling!
3) Cut your costs - I dont think you need to worry about this area quite yet, and I might be inclined to put your online shop on hold as there is still plenty of scope for you.
Where do people at school get their money from? do they buy at the end of the month or the start?
Is there a school newspaper you could pop an advert in or some flyers?
even a notice board.
How about the boys who deliver the local newspapers to your neighbourhood, could you get them post some flyers for you.
Im sure if you worded it well ( everyone loves a good story ) local boy 13 trying to start own business....... you might get some interest
Good post and good for you
Tommy