How to assign an Apple ID to our iPad via ASM

Not quite sure what you're asking. The Managed Apple ID is for a user so when you go through the pre-stage enrolment setup, you sign in as that user.

Have you synced ASM with your Jamf instance? You might need to import the users:
http://docs.jamf.com/10.10.0/jamf-pro/administrator-guide/Importing_Users_to_Jamf_Pro_from_Apple_School_Manager.html

It might be easier if you can tell us what you're trying to achieve? The Managed Apple IDs are only needed if you're using iBooks and want a way to backup the iPad to iCloud. Apps can be pushed out via VPP Device Licenses.

We can not do both.
Is there no way to recommend this to ASM or Apple also jamf?

The user would go through the setup assistant and sign in with the MAID. You cannot assign an Apple ID to a device - not through jamf, not through ASM. You could complete a feature request if that's something you want to see as an option.

It's best not to think about assigning Apple IDs to devices. Apple School Manager is broken up into three areas; Devices, People & Content for very specific reasons. A managed Apple ID identifies a person, not a device.

If you are planning a deployment, it would be best to have your SIS feed roster data (students, instructors & staff info) into Apple School Manager, and have the Managed Apple IDs created automatically from that data. Ideally, the SIS would provide Apple School Manager with the email address, so that when you create the sign ins for those new Managed Apple IDs, the user will get an email with instructions on how to sign in. For younger students, you may want to use a simple passcode, and set the initial passcode to a single value so each student uses the same initial pin (as directed by a homeroom teacher perhaps) and then is prompted to set a new pin when completing the sign in (this can be done at setup assistant on the device).