Here’s Mrs. Nelson. It’s a public school with both boy and girl students.
To die by one’s own hand is a longstanding reference to suicide; I would expect anyone familiar with it to easily make the connection to “blind by their own hands” meaning self-disoculation.
While the warning of masturbation causing blindness is also longstanding, it has always been in reference to continual behavior over a stretch of time. Note that Curtis says “by the end of the day,” not even enough time to need glasses :p, let alone go blind.
So no reference to masturbation, rather to the sheer psychic horror of seeing one’s stern, fat, ugly teacher wiggling in a bikini.