we remember[ ]the classroom after school

We remember [leaving] the classroom after school. 我们记得放学后离开了教室.
解析：
1. remember 用动名词作宾语表示“记起来做过某事”；remember 用动名词作宾语表示“记住要去做某事”.
2. 本题中的时间状语是 after school（放学后）,按照情理,应该是“离开了教室”,而不应该是“要去教室”.
3. 在没有上下文的情况下,括号中还可能填入其他动词,但不一定是动名词,如：
. * We remember [cleaning] the classroom after school. 我们记得放学后打扫过教室.
. * We remember [to clean] the classroom after school. 我们记住放学后要去打扫教室.
. * We remember [check]ing the classroom after school. 我们记起来放学后检查过教室.
. * We remember [to check] the classroom after school. 我们记住放学后要检查教室.