Is it legal to get a cannabis card in CA if you work for a public school district?

If a person works for a public school district, and is a member of a public employee union, can it cause problems if you apply for or actually get a cannabis card? The rules for the union are that there are no drug tests, but if you get convicted of a drug offense, you will lose your job for at least 5 years. I was just wondering if anyone has any experience with this. Can the school district find out about it if you apply for an receive a card, and/or take action against you?

My initial reaction is that it could be a problem, but the more I think about it, I begin to wonder if a person could actually AVOID a conviction (or even a charge) if one has a cannabis card to explain a small amount of weed (like less than one ounce). I'm hoping for someone with personal experience. Thanks!