Printing a 4x6 when not connected to a printer

If this school, or the school district has Information System staff, and Computer policy, then it is highly doubtful the teacher can modify anything. So, the first question to ask the author is so you have an Information Tech staff ir staff member that maintains the computers?

Typically no-one other than the I.T. staff/employee will be allowed to modify/move/install anything on the computers or the network. Not hardware, not software. 

Also, typically, no Flash media will be allowed, that just begs for malicious code, They may have allowances for SD cards in limited computers. 

"I am a high school photo teacher and we have one color printer and 20 computers. How can my students print from LRC when their computer has no printer installed?" I have two teachers in my family and know the problems that school board IT rules can cause which sounds like what you may be experiencing. As others have said, sharing the printer to other PC's is trivial and your IT should be able to facilitate this if you are locked out of Administrative tasks on the computers. This is not a LrC problem.

It sounds like you have determined that 'sneaker net' is viable. I suggest that knowing schools can also block USB access. and likely the best alternative of a bad situtation. As someone sad 'Print to JPG File' will allow you to get the image of as a file to move around. Instead of 'sneaker net' think about a shared folder in whatever the school is using e.g. Google Student.

It was as requested just a suggestion. If the computers aren't networked another option is to use WiFi to share the printer. USB Wifi adapters cost about $10-15 so a fairly inexpensive solution, easy to install, and most systems today are already equipped with WiFi.

If the computers are on a network you can setup the printer as a shared printer.

https://support.microsoft.com/en-us/windows/share-your-network-printer-c9a152b5-59f3-b6f3-c99f-f39e5bf664c3

I think that printing as JPEG is indeed the only option.