That must be very frustrating. You want your children to have everything they need to succeed, but school fees aren't cheap and they are necessary. I'm not sure exactly where you live so my answers may or may not be effective depending on your city, but here are a few ideas to try.
First, I would speak with the school to see if they have something in place to assist families. Chances are that this is not the first time this has arisen and they may have a way for you to offer assistance.
As well, I'd check with government agencies which support low income families. These agencies may also have options for you, but be forewarned that there will likely be paperwork to prove your income, etc. The same goes for checking in with your city hall to see what they can do for you.
Another alternative would be to check with your local community centre(s) - they are there to support and offer community resources. You never know what they may have implemented to assist families in your area.
There may also be businesses locally who support this kind of thing. I've seen businesses collecting backpacks and school supplies in September to contribute to schools who then give them to children whose families can't afford them - let's face it, school supplies can be super expensive!
If all else fails, phone your local government representative to express your needs and concerns. Ideally, this person will have some answers or at least be able to find out the answers to your questions.