The school with ClassApp has their entities IDs. Therefore, for integration works well, you might need to build some relation table to associate your IDs to ClassApp IDs:
|Student||School ID||School ID (ClassApp)||Entity ID||Entity ID (ClassApp)|
Student at classapp calls Entity with type Student. To import a Student at ClassApp, you'll need to collect some datas:
To send a message using ClassApp API, you need to understand Entity and Tag associaction.