From my own Civil service experience, it looks very suspiciously to me like the school either pushed him back too soon into “normal” teaching 5 days a week or he was so keen to get back to the job he loved that they caved in. We had a very strict routine for any long-term sick, as did Sarah at Plymouth City Council(Local Government & different terms & conditions from Civil service).
In both cases, you would start back at around 20 hours a week instead of 37. After 4-6 weeks, you go on to 25, then 30 after another 4-6 weeks & then, after around 3-6 months, usually the former figure, you restart full-time work.