Is it illegal for my parents to withhold my medication?

I (16m) have recently been prescribed with ADHD medication. My medication is one that I take in the morning, then a second pill that I take up to twice a day as necessary. The pill to be taken every morning is not an issue, as it is given without argument. However the pill I am to be taken to top myself up is withheld from me. My parents keep them in a location not disclosed to me and allow me ONE after school, not when I’d need them, which is when it’s be best as the morning pill starts losing it’s effect typically halfway through my school day.

Is this allowed?