Bitrix24 Alternatives

I've been using Bitrix for the last few years to manage my small business but their new pricing, specifically that extranet are included in your user count, have priced me out.

I have ~20 employees and ~500 clients at any one moment. I'm looking for something with the following criteria:

1. private streams / projects for students. important features are secrecy (only invited people can see), ease to make posts, easy to scroll through & count lessons
2. an app
3. a storage system
4. streams / projects only for employees
5. flat rate OR rate for employees only
6. calendar system

Extras:

1. good video call feature where we can supply links to people who have not signed up for the CRM
2. internal links: easy to provide link to posts, documents, etc.

We work in education, so anything with an education focus would be appreciated.

Thanks!