drop-in 和drop out 什么意思？

drop-in 和drop out 什么意思？ I dropped out of Reed College after the first 6 months, but then stayed around as a drop-in for another 18 months or so before I really quit. So why did I drop out?
第一个故事讲的是点与点之间的关系。我在里德学院(Reed College)只读了六个月就退学了，此后便在学校里旁听，又过了大约一年半，我彻底离开。那么，我为什么退学呢?