in came a stranger__our class(interrupting)
这是个倒装句
其实是 A stranger came in interrupting our class.
interrupting our class是表示结果的状语成分：此人进来的结果是打断了我们的课堂学习
用interrupted 肯定是错误的,因为这句话已经有了一个谓语动词 came,如果选填interrupted,那么其前方一定要加个and 才能使两个谓语动词友好相处,即A stranger came in and interrupted our class.
否则就是病句.