中学? middle or high school 中学
seems to be both terms for middle school (junior high school)
and
high school (senior high school).   

how does 高中 and 初中 relate to these words also?

http://www.linedict.com/dict.html#/cnen/entry/f9a933f7897246cb984b10f7c24a40d3

Describes it as middle school

http://www.linedict.com/dict.html#/cnen/entry/20524e88708646beb309d96176102585

Describes it as high school

maybe it depends on the region (China, HK, America, etc) where you attend school?
初中 is the last three years of the nine year compulsory education. 高中 is high school or gymnasium. 中学 comprises both of them, altogether six years of middle school (as opposed to primary school and higher education).