He was a magician in his hero days, and magician normally doesn't have high physical. Also any significant muscles he had would be gone after being a common professor for 20+ years. Now he's much older with usual old man's weak and aching muscles, that a massage affected him so much.To be fair, it's never hinted that he ever lost his strength from his hero days. Just that he's been relegated to a biology teacher in school.
Just wait for the highly likely physical education chapter where the other teachers try to embarrass him and he beats them all without even trying.
Though I'm sure he can still rocks in fight, because he's a magician.
He said he failed to meet their expectations, though it's unclear if it's about his strength or about his stance regarding monsters (he's against humans killing and experimenting on monsters). Though he needed to be strong enough to fight his higher-ups to save that spider. They even allowed him to do his teaching works afterward.failed because he was weak
So I highly doubt that his self-evalution (only an old man that can use a little of magic) is actually accurate.