Which would you rather be — consistent or flexible? Is it possible to be both?
Here is an example most parents will recognise: getting young children to sleep at a consistent time. When children are very young, we have little control over when they go to sleep. The only thing we can control is the time we put them down. We are establishing a routine — bath, story, sleep. Repeat.
As our children get older, we can set expectations around bedtime. But what about the nights when friends are visiting or it is a birthday? Is it acceptable to break the routine? Yes — as long as there is a routine to break in the first place.
Some parents try to be flexible with a routine before the routine is established. You cannot do that. You are simply choosing flexibility over consistency.
The same principle applies in the classroom. It makes complete sense to establish routines and reasonable, consistent expectations. Once you have done that — really done it, not just intended to — you have earned the right to be flexible. The flexibility has meaning precisely because the consistency already exists.
Consistency first. Then flexibility. Not the other way around.