Can I return to Thailand on a tourist visa one month after completing a 9-month education visa?

Yes , you can go and just you have to keep phone number of any university authority , sometimes Secondary immigration ask for their number, so be prepared. I was in same trouble, When i went again on 2 months tourist visa . In any problem you can direct tell them that please you can talk with my teacher , I have some important work at my university or collect certificates, but you have to discuss everything with your teacher first and keep all the required documents like hotel booking ,cash and return flight ticket they check this all this things in this types of cases and dont tell that you are staying here for more than 15 days.

I did this last year with no problems.. but as you know it’s down the immigration officer 👍

I'd say it's risky and you have a very high chance of being pulled out of line and grilled at the least. Denied entry at the worst.