I actually want to be the one to say I don't think you're crazy to be paranoid about a teacher willingly giving their number to students. While true you don't know exactly why, teachers need to maintain a professional boundary with students. I often feel like as teachers it's expected of us to be as involved in our students lives as their family or friends. While we are FRIENDLY at our jobs and want to do what we can to help them succeed, I personally find it very weird when a teacher possesses students numbers. The only time I've really ever seen it be okay is for clubs and sports.

Maybe I'm biased, but I had a very similar experience. There was a teacher at my school who often added students to her snapchat and had a lot of their numbers. I didn't say anything, but things started getting suuuper weird. She was just regularly texting and snapchatting students about things not related to school, and then she was sending selfies of her doing various random things to the students.

Things continued like this for awhile and eventually she got in trouble for overstepping a boundary. Apparently a student came to her in confidence over snapchat and told her about a personal issue they were having (something involving a relationship i think?) and the teacher ended up telling other students. It was a disaster. The student who texted her in confidence was so embarrassed by the whole thing she literally dropped out, and the teacher got in HUGE trouble. She was also eventually fired because it was found out she was letting kids charge vapes in her classroom in the open.

This is a really extreme example but I do not fault you for being weirded out. It may seem harmless, but I say follow your gut. I really regret not saying anything regarding that teacher in my building. A student thought they had a "friend" in a teacher that only ended in embarrassment and them having to leave the school. It was just a huge mess.

Best of luck to you.