If u're with a private gynae, u have better chances of getting HL. I am in the education line too, a pri sch teacher. I got 4 wks HL due to asthma and my colleague got 5 weeks due to GD. Of course it depends on the severity of your condition.
I believe gynae can only give if there are complications in your pregnancy. Btw, you can only start your maternity 1 month before ur edd if i not wrong.