This SAGE paper may help to clarify the implications of re-opening schools. The first 14 pages are clearly written and do touch on the concerns you have mentioned.
On the other hand, one detail that sounded ominous to me was that the "consensus view is that the opening of primary and secondary schools is likely to increase effective R by a factor of 1.1 to 1.5..."
assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/963381/S1102_Children_s_Task_and_Finish_Group_update_to_17th_December_2020_paper_on_children__schools_and_transmission.pdf
If you could direct me to a link demonstrating that school and support staff will be vaccinated before the 8th, as you mentioned, that would be much appreciated as it would surely make a difference.