New Title: explanation

My contract says that my workday starts from 08:15 and ends at 17:00. School is usually out by 4pm, but most teachers stay at their desks until 6 or 7pm. I have no idea what they stay late everyday for, but leaving the staff room "early" is a daily guilt trip for me.

Me : お先に失礼します
-I'm sorry (lit: I'm being rude) for leaving early.
All: お疲れさまでした
-You have exhausted yourself.

Formalized, just so my exit can be properly processed by all.