All teachers want their schools and classes back to normal - and they want that to happen sustainably, so that there is as small a chance as possible that a grand opening will be followed by a wave of school closures following outbreaks, or worse, a second peak.
In primary, IF the number of cases in the community is small, and IF money can be found for shielding teachers to be covered and cleaning to be stepped up, then increasing the bubble size is a sensible way forward.
No assemblies, staggered starts and breaks, reduced contact between bubbles, frequent hand washing - ie all the measures currently in place for the first wave of opening - will help. And if a case occurs within a bubble, then only that group is affected.
Secondaries cannot follow a bubble model, and staggered starts get very tricky with teachers working across different year groups. 1m social distancing only allows half the students in a classroom. So a full opening, even with handwashing, masks on transport to school, and increased cleaning, basically makes the whole school a single 'bubble' for transmission. So a few cases can close down the entire school, not to mention affect those sharing public transport with pupils. That does increase the chance of a rapid sequence of openings and closings in those areas where community transmission is higher.
OPENING schools is perhaps the easy bit. KEEPING them open, through ensuring the smallest possible number of cases, is harder.