Your mission, should you decide to accept it, is to get yourself and a child:
1) out of this school
2) into a vehicle
I'd have the kid leave under his/her own power. I'd leave at the time when there were a lot of people coming and going. I'd say that I left significantly later than I actually left, relying on the invisibility of being in a crowd to conceal the real time I left.
Easy to do if the child is mine.
If the child is not mine, then I would tell the kid that I needed some help and the teacher/principal/his mom told me it was okay for him to help me. I might make the offer more enticing by making it something the child might really enjoy seeing, like an electricity project.
3) drive away without anyone seeing the child in your vehicle
Possibility one: if this were my short kid and tall vehicle, I'd just have the child get in as usual relying on the obscured view into the vehicle to conceal the child.
Possibility two: I'd spill something on the floor and tell the child to mop it up while we left the parking lot.
Possibility three: if this were not my child, I would open the vehicle door or trunk, then take a look around to see if we were standing in anyone's line of sight. With the prevalence of SUVs, vans and other tall vehicles, there's a good chance we'd be hidden from direct sight. I'd either hit the child over the head to cause unconsciousness and stuff them in the vehicle or just stuff the child into the vehicle still conscious.
Parameters:
a) there are many people entering the school as you do this, both adults and children. Keep this in mind, because you must ensure, or minimize the chances of, anyone seeing the child with you.
I'd be relying on the invisible nature of being in a crowd. There are lots of kids around, lots of adults around. If I acted relaxed, assured and avoided close contact with anyone, I'd assume that I would just fade into the background.
b) there are many people exiting the school as you do this. Most are adults; few, if any, are children. Keep this in mind because the child you need to get into your vehicle will stand out as a rare child exiting the school. You must ensure, or minimize the chances of, anyone seeing the child with you.
I'd leave while there were still a lot of parents arriving with their children. From having attended many such events with my nephews, I know it's never a homogenous one way flow; kids forget things, parents forget things, there's too much to carry in a single trip, etc. There are always people walking against the flow, both adults and children. I would just make sure to leave while there were a lot of people coming in with their children.
I would say I left later, to help confuse things. At such an event, I would be betting that it would be unlikely that someone would notice me and check their watch to establish the time they saw me. They wouldn't have any reason to because nothing had happened yet.
When most people check their watches, they are not actually looking at the time, they are checking an interval ("have I been here long enough" or "how long do I have until I have to go?"). That's why, if you see someone check their watch and then ask them the time, nine times out of ten they have to re-check their watch.
So if I claimed to have left later, I would rely on my claim confusing people's memories.
c) the child may or may not be seen for a final time at the south entrance of the school
I'd claim I had last seen the child near the south entrance but I would not leave via that entrance myself. Since the classroom was near that south entrance, anyone who remembered seeing us would be more likely to remember seeing us there if I deliberately placed myself and the child there.
Leaving through a different exit could easily be explained by my child having forgotten something in our vehicle.
d) the vehicle may or may not be a Ford truck or a red Mustang
So long as it wasn't something that would really stand out, like a Hummer or a DeLorean, I wouldn't worry too much about my vehicle. It would be just one of literally hundreds there that morning.
e) you may or may not have one distinctive feature that would normally be very noticeable to others - very long, curly red hair.
I'm not familiar enough with the Portland area to know if a ball cap would be normal wear or not. If I had long, curly red hair, I'd pull it back in a ponytail or braid it. I'd be going for the generic soccermom look.
f) the has one distinctive feature that would normally make him more memorable - he wears glasses. Only approximately 10% of children wear glasses.
http://www.dictionaryfordads.com/childrenandglasses.htm
I think I'd just leave the glasses be. Or I'd tell the child they were smudgy, let's go to the car and clean them up. This would also serve to get the child out to the vehicle.