https://smenarna-praha-1.cz/en/contact/ was the place with the best rate I found in Prague. Using Google translate I was able to haggle the rates right down to almost interbank, and in one case better than interbank. There was a Czech person exchanging about 10000 euros in €500 notes. I appreciate a 15-year old who hasn't travelled much may not be confident at doing something like this.

Shops / cafes / stalls in central Prague targeted at tourists accepted euros at 1 to 25 the last time I was there, but the rate is now 23 to 24, so it will probably be 1 to 20. But even if paying in CZK those shops probably charge twice as much as what you "should" be paying.