Depending on the settings set by the administrator for you Webex site, whether it's a Personal Meeting Room or Scheduled Meeting and if the students have accounts on your site or not, you can change how people can join the meetings. I've attached a screen shot showing the CoHost options for Personal Room Meetings. You will either want to disable this or set it to the first option "Let me choose cohosts..." If it's a scheduled meeting, guests are anyone that don't have a Webex account with your organization. You will want to select Guests wait in the lobby and possibly lock the meeting immediately. Hopefully this gives you what you need but reply back to this post if you have more questions.
