My children are both autistic. I have arranged to pick my youngest up 10 minutes early each day. I park in the car park across the road, go into the school, get him, grab him by the collar and don't let go until he is in the car. Then I go to my other son's school, luckily they do have a car park. But it is very busy indeed! I have to get out, grab my younger son (he can't be left in the car alone) and keep hold of him by the collar as we go across the car park to collect my other son. Who I then grab hold of with my other hand and escort them both back to the car.
2 bolters - 2 hands - no problem 
How it would work with more than 2 if they were not able to walk alongside and follow instructions, I don't know. You'd probably need more people! But you indicate that only one is a risk, so it sounds doable.
Or you could arrange for the child's TA to walk him out to the car.
Or - where do the teachers park? arrange to have handover there?
Wrist strap. I used to have to use special wrist straps for years with mine. It meant that they couldn't run off.
It's tricky, I give you that. But there are other solutions, you just haven't thought of them yet.
And I am not having a go! You asked if people had ideas, I am offering a few.