I had it very badly as a child and spent nearly 2 years in plaster casts (6 weeks in, 6 weeks out) as that was the treatment in fashion at the time.
I managed to break just about every cast they put me in - even the fibreglass ones for racehorses - as it wasn't painful once the cast was on.
In the end I got a superb zip-on plaster cast - it was like two pieces of stocking that they filled with green foam and rolled out, then fitted it round my leg, zipped it up and left it to harden. That way I could wear it all day but take it off for swimming, baths, sleep etc. No idea if it still exists but this could be a solution for you.
Otherwise the best things I found were shock-absorbing sorbethane insoles in all my shoes and exercises to lengthen the muscles in the back of my legs.
So sorry for all DCs with this horrible disease, it is immensely painful (like having a chunk of broken glass stuck in your foot) and very tedious. I hope schools are more sympathetic than they were back in the 1980s.
It's very hereditary - my father had it badly, and my brother and one sister had it very mildly. I'm praying my DD doesn't develop it as she's very keen on dance.