Instanced realms cause incorrect context on student formspec list

[pauldpickell]

Describe the bug When an instanced realm is created by a teacher player, the context for other players is incorrect. When selecting classrooms to teleport to from the student notebook, the selected realm and teleported realm are not the same.

To Reproduce Steps to reproduce the behavior:

1. Create an instanced realm as teacher player
2. Another player joins the server
3. The other player opens the student notebook and navigates to the classrooms tab
4. The other player selects a classroom from the list and clicks the teleport button, but is brought to another classroom

Expected behavior Instanced classrooms of a player should not influence the context of other players when selecting classrooms.

[pauldpickell]

solved by #231