The health protection agency used to advise staying away until you know you were immune.
I've just checked both patient.co.uk and the replacement for the HPA and it just says contact your health care providers.
It's annoying as it's very anxiety provoking. My mw was dismissive of it all but the gp took it seriously.
I'd self certify for now and go see your GP. You could legitimately say you are extremely anxious about contact. Antenatal anxiety is a very real and horrid thing - I developed it in my first pregnancy due to this. I seem to remember my union being supportive.
It's worth remembering that the worst case scenario is catching it from your own children at home - this is more likely. However, I'd been teaching over 10 years with Sen children (lots of close contact) and never caught it.
Nice guidelines now say this:
*"Advise the woman to avoid contact with other pregnant women and people at risk of complications, until she is known to be uninfected, immune to infection, or no longer potentially infectious.
The infectious period is 7–10 days before the rash (if any) develops, until one day after the rash appears.
If there is any uncertainty on the recommendations on isolation from other at-risk people, contact the local virology, microbiology, or infectious diseases department for further advice."*
It's avoiding the previous advice to stay away till you know you're immune however tells the pregnant woman to stay away from other pregnant women she could infect till she knows if she's immune or not. Which kind of indicates by omission that the children she teaches could be potentially infectious.
cks.nice.org.uk/parvovirus-b19-infection#!scenario:1