While this post remains open, it is relatively old. Hopefully, the poster has already found alternative employment. Nonetheless, I wanted to offer a couple of points which will hopefully be helpful to him and to others in similar circumstances.
The principal did not appear to violate the law by suggesting that you resign. Unless the situation at work became intolerable, voluntarily resigning usually undermines an employee’s ability to seek relief subsequently against their former employer. (If you worked for a public school, you may have had civil service and/or collective bargaining rights available to you. In contrast, if you worked for a private school, you were probably hired as an “employee-at-will” and could have been terminated without good cause.)
With that said, in addition to finding or starting a new job, you may have a moment of introspection about what happened at your old job. Are you certain your behavior did not change for the worst after you came out of rehab? Was there nothing in your post-rehab conduct which could have made your teaching colleagues and/or the students uneasy?
Of course, it is possible other teachers, administrators and students began to treat you differently simply as a result of their hearing rumors about your rehab stint. It is also possible, though, you did change deleteriously in a manner you did not perceive.
You may want to consider doing some objective “soul searching” to determine whether your conduct contributed to the deterioration of the work environment at your former employer. You should consider doing this one your own and/or in a therapy setting.
Wish you well in the future.