Is there any regulatory guidance on the amount of time you need to back CONUS before going to a school?

Make sure you get all of this in writing. Would hate to see you lose an opportunity because of the deployment and the chain of command verification. I was in Hawaii and got a school semester for reenlistment. Found out that a colleague got the same thing, but due to work staffing issues, he was unable to go to use his college semester and lost out on the opportunity. I enrolled myself in school two days before I reenlisted, so that I would not get the same treatment. That semester launched me into my officer career.

Are you going to School TDY enroute or TDY & return?

That will drive the timeline.
No there is no regulatory guidence I know of... other then, they "should" provide enough time to set you up for success. You are a known loss, its not news or a surprise, whether they lose you 80 days into the deployment or 170 days in its the same known loss.
Assuming you are TDY & Return you remain on the units books.. that will affect there assigned vs deployed numbers and be cause of hand wringing. Behind scenes to you that senior commander is dealing with a DOD or higher HQ deployment order that requires XX% of assigned strength to be in country. If they are at or below min numbers, you leaving may very well drive a requirement to find another to deploy.

Help yourself help the command team make the call.
Figure out oif you are TDY enroute or not, then sit down and make a detailed plan of what you need to to do to get off post and too school.... DON'T FORGET DTS..likley a 30-45 day process... it is NOT something you can wait till you get back to start... Ideally, the DTS process shoudl start as soon as you have orders for the mission, be it a school, or other TDY tasking. Earlier is better.
So make that plan, figure in a reasonable amount of time/days overage for shit happens stuff...car hits a power pole and the G3 section is down for two days, sorry no DTS processing going on, no S1 processing of orders.. DOIM pushes a "patch" and the network goes down for three days....you get a flat tire and miss your out processing appointment and have to reschedule 2 days later.
PLan that timeline, be meticulous but reasonable on the times allotted, and present that plan to the command.. let them "see it" and visualize why the 22 days is needed.
Otherwise its just abstract numbers... And 10 looks good, 12 is being "nice" 15 days is just letting you screw off, and 22 is just pushing dereliction of duty ...but if presented with what you need to do each of those 22 days,,the buisness hours of the shops you need to visit, the DTS, G1, S1, G3, S3, ect... they may be more inclined to work with what you need.
Bottom line, Make it "easy" to say yes, give them a reason to say yes, give them the "easy" solution to decide upon.

In don't think there is any regulatory guidance to a latest possible date because it's common knowledge not to send someone on a deployment when they are scheduled yo PCS at that time. Basically they have to give you enough time to clear, which isn't a lot. This is really a command issue and I suggest you immediately elevate it, especially if you have a family that you need to square away. But, you still need your orders and to submit for your manual 1610 prior to school.