Because, you know, the government has a habit of landing all sorts of shit on schools at the very last minute and that is unacceptable
Agree completely. We’ve seen this repeatedly throughout the pandemic. Knee jerk, poorly thought out, last minute U-turns, disorganised. Leaving everyone confused and/or floundering. Including schools, parents, teachers and students.
Why do schools have to be spoon fed plans by Government? spoon fed 
Because schools as publicly funded institutions have to follow and therefore wait for government guidance.
Many HE institutions are moving fully back to f2f teaching in two weeks because govt guidance says this is okay. Hundreds of students crowded into a single lecture hall. Most built without windows. Thousands of students flying back from all over the world.
On the one hand, it’s bit of a gamble that a PCR on entry and isolation for 48 hours is sufficient to not spread Covid further. On the other, if govt does suddenly bring in new restrictions like they did last year, then the travel plans of these poor students will be thrown into chaos.
Johnson hates to make any commitment decision. He knows that someone will be unhappy with him whatever decision he makes. So waits until the very last minute to make them. What a way to run a country during Brexit and a pandemic.