I wouldn't.
1. I don't like kids. I dislike most of them actually.
2. I typically average like 55-60hr a week on duty already. (Work a bit of security).
3. There is no I'm only here for an active shooter/threat. When sh!t happens you are going to have to handle it because they are going to know you are a cop and they are going to turn to you to solve their problems. With essentially means you'll be dealing with typical BS. Fights, irate parents, belligerent students, theft, students carrying guns/knives on the school property, students selling drugs on school property, parking lot crashes, etc. (That is the type of stuff our school resource officers handle on a frequent/daily basis).
4. 95% of our schools have SRO's with my agency already. (Cept for private schools)
Now, would I be interested in working paid ODS at one of the private schools with no SRO? Sure. $40 an hour sounds bout right. I'll be there every day I'm off shift.
Where I live there is a lot of paid security gigs which means not many officers are going to do anything for free here.... They can just as soon be making their OT rate at any other location.
A better suggestion/method would be have an office which they can park near which has a lounge area, restroom, TV, WIFI, snack/drink machines/ microwave/sink/coffee pot where on duty LEO can go to hang out/eat/type reports. And greatly encourage on duty LEO's to hang out at those places when not on calls during school hours.