I ALWAYS send in reply slips/consent forms/school trip money etc on time.
Now that DS is in Secondary school the responsibility to actually hand the slip in lies with him. He's pretty good, but sometimes things will stay in his bag for days, and occasionally beyond the last day to reply.
What do teachers think? That the parent didn't give it to the child in time or that it's been sitting in the child's bag for a while. Or maybe they just don't think about it at all!
While I'm here, I have to say I think secondary school teachers are brilliant. I was so impressed at my son's first parent/teacher night.