1: Be sure your School is signed up with Zoom as a school, this can be verified here: https://zoom.us/education click the requested offer button. Doing so puts all school related accounts (email addresses) under a special privacy policy that is more comprehensive than the general public accounts. Please refer to: https://zoom.us/docs/en-us/childrens-privacy.html for more specifics.
2: Always use your school account for your zoom meetings and only invite students via their school account also, so all meeting attendees are protected under the children's privacy policy as in #1.
3: Keep your programs and apps updated, in zoom the update button is found in the profile button upper right corner of zoom application.
4: Always set a password when creating the meeting. It doesn't need to be hard, just avoid using super simple ones like: 'password' or '123'.
5: Only invite the people you need to have at the meeting. Send the invite in email to the meeting members only, avoid posting small group or classroom meetings on FB or other social media. The more people you share the link with the more you risk unwanted pranksters, if one of the attendees passes the zoom link on to a prankster then they can disrupt your meeting. This is a vulnerability of all video conferencing tools.
6. Check your sharing settings: click the white up arrow next to share choose advanced as shown in the image below, typically only the teacher (host should share). This will block the zoom room from being interrupted with inappropriate images and videos.