Im in Ireland so theres no CMS and back to school is a really expensive time- its not just uniforms, its the book rentals/purchases the not at all voluntary fees and every other cost and charge you can think of.
Maintenance is classed as day to day expenses for the child and unless you have come up with an agreed figure between both parties, a court will order a reasonable sum (15-20% of salary I believe), they believe in you make 'em, you pay for 'em.
Back to school and school costs, Christmas and birthdays are seen as exceptional payments, so my order states that back to school costs are to be split 50/50, BUT sending a child back to school in Ireland is more expensive than the UK. Its not just uniform, its book fees, book rental not at all voluntary contributions and the list goes on.
Christmas and birthdays are somewhat of a greyer area as it depends on how much the maintenance is, and also the value each parent places on the importance of that holiday- so ex and I just do those events separately. I don't want to finance gadgets for the kids, and he thinks that I am being stingy when I buy them books. Its easier like that.
School however, is non negotiable and it is ridiculously expensive, so yes, I think that the initial back to school cost should be split as an additional expense not covered in everyday maintenance. However, if my children need new runners in the term, I dont go running to him demanding he must pay 50% as an ongoing cost. That's wear and tear, I just need a hand with the initial outlay when it all comes due at the one time.