Don't use mints and it is best to just brush with water only while fasting too. And as long as you don't overeat you shouldn't put too much weight back on. Your body naturally tries to regain a healthy weight after fasting. If you are still overweight and you refeed properly the body will only ingest what it absolutely needs and once having done that will probably keep on losing. If you have lost more than is healthy then you will regain weight until your body is it's natural healthy weight again. So just refeed the normal amount of time without overeating and you'll be fine. On the other hand the refeeding diet is a very healthy one and can be continued indefinitely if you wish.
There is no knowing how long your discomfort will last. Some have lots of discomfort during a fast, some have very little. And some can exercise throughout a fast while others are bedridden most days. There are no guarantees either way. You can plan to school and do PE throughout your fast, but be prepared to rest at any time if your body needs it.