What does middle and high school mean

It has to deal with middle school and high-school

I’m British
Wee don’t say that here

well middle school is for 11-14 year olds and high school is for 15-18 year olds
(doesn't apply in the country i reside to but for me 11-12 year olds are in sixth grade (primary school) and 12-18 (21 if you're in a special class) year olds are in high school.)