I think as soon as serology tests become widely available and cheap, lots of people will get a "certificate of immunity" and be allowed/encouraged to return to work. My thinking is that in 3 months (i.e. around end of June), lots of people will be back to work.
In the US in particular, public schools get money from the federal government conditional on administering state tests. It doesn't look like the stimulus bill is leaving them off the hook, so schools have a very big incentive to give the test by the end of the school year (around 25-June). There was no official statement that state exams are cancelled for this year, and what's more my son's teacher resumed the exam prep with the kids yesterday.
So, I'm not sure when stores will open, but I give a more than 50-50 chance that schools will open at some point before the Summer vacation, maybe second half of June or early July, in order to give the state exams.