drop in and out of。。。。。。。。。 啥意思啊

drop in :顺便拜访一下。
drop out ： to withdraw from 退出。
如：He has decided to drop out of politics.
他已经决定退出政坛。
Seven students dropped out of university last year.
去年有七个学生被退了学。
找到的句子:
Last chance high :how girls and boys drop in and out of alternative schools

要放在句子里才翻译得通,没有背景,只能在上面找找了