Somehow, you have applied Adult Content Filter Profile, and you experience some URLs blocking.  Please note that the Adult Content Filter Profile is based on Apple's filtering logic. But you can set exemptions to allow URLs. 

There are 2 steps:

1)  Setting > School Policy > URLs > Permitted URLs > Please provide URLs which you want to access

2)  Device > Device Management > Bulk Actions > Advance > Set School Policy > Select Devices

Tips：

Use main domain instead of full link will cover sub domain, eg:

eClass.hk covers

Www.eclass.hk

Www1.eclass.hk

Sub.eclass.hk