Flat, supportive shoes. Heels are for SLT to swoop around in, not for anybody who is on their feet for 10,000+ steps every day - you can do 3,000+ by 10.30am with trips to the photocopier and general moving around, never mind taking messages or if the photocopier is upstairs/the other end of the building.
Other than that, it varies greatly. Always have a blazer on the back of your chair in case of visitors if you don't actually have to wear one at all times. Seethrough, sleeveless or spaghetti straps are generally a bad idea. Until you're there and see what people actually wear, stay clear of above the knee skirts, very light coloured trousers, very wide or very close fitting ones and leopard/animal print; the latter will often only be accepted if the HT/Deputy wears it to work herself. It's good to have a formal style coat in case the fire alarm goes off, rather than a puffa/anorak/hoodie.
My safe bet is for a dress (I don't do chintz, ditzy or other prints, I'm a knitted/houndstooth/block colour fan) that goes to my knees or lower, skirt and top or tailored trousers and a softer shirt or top. They've been happy with a tweedy pair of trousers and a pale turtle neck as well.
Bear in mind that the most casual a man may be able to do is a Tattersall shirt, jumper and tweed blazer (ie, full on country casual). And many will have to wear at the least a standard blue or grey suit and jumper over a plain shirt - if it's a fancy academy, it's more likely to be very strict, as they piss parents off with the price and rules for the pricey uniform, so think the parents won't be happy if their staff aren't kept to almost identical rules to the children. Faith schools can also often be a little more formal. It might sound shit, but I suspect that for some places, what they actually want is Kate Middleton on a relaxed official visit with Kate Middleton on a walk with the dogs for INSET days. And some expect Kate Middleton at Prince Philip's funeral every single day. Others are fairly relaxed daily but expect you to frock it up a bit (no, skirts aren't compulsory, I mean look very smart) for official events, celebrations and anytime there might be a VIP around.
It's not what I'd wear in my own time. But it's not my own time. It's work time.