The easy solution for your question is to install and use the Hot Question plugin. It allows students to post questions/topics/suggestions, read what others have posted, vote for them, the teacher can set priorities, and the teacher can set it so posts require approval before they can be seen by anyone. Teachers can start a new round at any time, remove inappropriate posts, remove rounds, use groups, and so on. If someone persists in inappropriate posts, the teacher, or admin, can download the posts to save as evidence, and then remove as necessary.
I maintain this plugin, and would be happy to let you try it, but I just shutdown my server to do a rebuild. CPU fan is making funny noises.
I'm in the process of creating an alternate place for trying it out...let me know if you want to try it.