Answer by Professional Japanese Teacher
Minasan ni gomeiwaku o kakeru kamoshirenai node, jishu kakuri de, gakkoo o oyasumisaseteitadakimasu.
I’m sorry for the inconvenience caused but please allow (my son) to take leave from school because (he’s) on self-isolation.
It must be a trying time for you and your family. “Jishu kakuri” literally means “voluntary isolation”. When you are on self-isolation or self-quarantine, you may unknowingly cause some inconvenience to your colleagues or teachers.
In this case, you can use the second phrase to explain the situation to your son’s teacher. I hope your wife will recover soon and everybody stays safe.