Out of interest what is it that you think means we should be less precautionary in a classroom than a shop?
There are very few (if any) downsides to asking adults to wear masks in shops or on public transport. The trips are usually quick, short-term discomfort can be managed so all-in-all it is a low price to pay for the potential "low to moderate" reduction in virus transmission based on "very low/low to moderate" evidence.
Masks in classrooms have got potential negative consequences that cannot and should not be dismissed without further consideration. Violetlavenders has listed out some of them on the previous page. Copying and pasting:
However, covering the lower half of the face reduces the ability to communicate, interpret, and mimic the expressions of those with whom we interact. Positive emotions become less recognizable, and negative emotions are amplified. Emotional mimicry, contagion, and emotionality in general are reduced and (thereby) bonding between teachers and learners, group cohesion, and learning – of which emotions are a major driver. The benefits and burdens of face masks in schools should be seriously considered and made obvious and clear to teachers and students.
People will have different opinions on importance of "emotional mimicry and bonding" in comparison to the risk of Covid but it is not (and should not be) a straightforward decision.
PuffinShop also posted quotes of several teenagers who were asked to temporarily wear masks at school. Some were fine but some were not. We should not be dismissing the feelings of the latter in the context that overall benefits of masks are relatively marginal.