I second what 2GiftedKids says about separation anxiety, and add that you want to avoid what I've come to call caregiver-induced separation anxiety.
2GK kinda touched on it with the idea of separating quickly and not hanging around, but there's more to it than that. It's how you treat the separation in totality, from talking about it well before hand, all the way to the act itself. Throughout the process, there are opportunities for the caregiver to induce anxiety in the child, by exhibiting or exaggerating their own concerns.
Imagine what goes through a toddler's mind when they hear their primary caregiver say, "I'm going to leave you here for a while. Are you going to be okay?"
"Am I going to be okay? I don't know. Why wouldn't I be okay? What's going to happen to me here? I DON'T KNOW! I've never been here before! I don't know any of these people! Are they going to do something bad? Whenever something bad happens, it's always my caregiver who takes care of it... but that person won't be here!! OMIGOD!!! PLEASE DON'T GO!!!"
The fact that you're already thinking about your DS experiencing separation anxiety means you're at risk for inducing it. It's critical that you keep any concerns from leaking out in words, tone, body language, etc. Keep the mood on the positive, and ensure he knows:
1) There are adults present who you trust to keep him safe.
2) This is a normal part of growing up, and you did it, too.
3) You'll be there when the school day ends.
And, only if he asks, I'd let him know that if there are any major problems, they'll call you, and you'll be right there.
