Highheels, I totally understand your dilemma as a secondary school teacher myself. My children were very well behaved and didn't actually ask whether I was pregnant before I told them, but I was sure that the older ones were wondering. I didn't tell my classes until I was 16 weeks, but that was only because half term and the telling-of-important-people-face-to-face got in the way! I actually only told my tutor group, sixth form classes and Year 11 class properly, and the rest I left to the school grapevine (which was very effective!). They've been very sweet ever since, helping to carry books up the stairs and running errands to the office so I don't have to go. I'd say tell them if you've told everyone else that you want to know, and enjoy all the random questions!
I'm just getting over a horrible virus which knocked me for six, so sending lots of sympathy to halo for your cold, but went back to school today and survived so must be over the worst of it. One of my tutees (not the brightest little button) asked if I'd been off school (for 4 days) to have the baby! 
Needing news from OneOf8, but so excited about your One Of Each, crazyd. Rather jealous, in fact! Sorry about your SPD, though - sounds awful. I've only had it mildly and been able to keep it at bay with pillows at nighttime and a bit of bouncing on my gymball of an evening.
Exciting to tell the parents, slowshow, isn't it? My father had always insisted that he didn't want to become a grandpa until he was 60 - my EDD has him at 60 + 6 weeks! Teehee. Even though we told my parents really early, I was too scared to tell any other family (or allow DH to) so left that until the evening of the scan... I was really bloated at the start too, so if you haven't already got one, get a bumpband (I got mine from Mamas&Papas) as you can wear your normal trousers but leave the buttons undone!
I've been making a spreadsheet compiling what all my various resources say that we need to get for the baby. It is a scary beast! Am hopefully going to get some hand-me-downs from my HoD, as they're not having any more. Ah well, pay-day today, so it might be the month finally to buy the nursery furniture...
Congratulations to roundtable and please don't worry about your cycle length - a line is a line is a line. Have you done a CBdigi? I found the words were very reassuring! My cycles were ridiculous - I'd just had a seven week ERTD/spotting and, according to the dates, conceived only just after that stopped. You could always ask about a dating scan when you have your booking in appointment with the midwife...
7 and a half weeks (plus a week of half term) left - very exciting, but I have SO MUCH to do with my classes before I abandon them to the unknown. I'm stopping at 36 weeks - just don't want to lug myself around school when I'm enormous!
to crazy - tell us more about your new house! :)