ALT dress code?

Assistant Language Teacher for a public school... I'm a woman and working there soon.. what's the dress code?

Ask the company|school you will be working for, but "generally" speaking you can't go wrong with business casual.

I used to be an ALT and for a while I worked as a substitute so saw various schools...until you know your school I'd dress up a bit; a casual skirt is good for a start.
My experience was that the elementary schools' teachers dressed more casually as the classes were more physical. Junior High was a mix, but still, pretty casual, but I had one teacher ask me to speak to another ALT about her wearing tank tops to class, they thought it looked kind of...skimpy. So I'd steer away from sleeveless.
Last, High Schools varied wildly but were the most dressed-up. More upscale urban schools had more upscale urban dress - guys wore ties. Women in skirts. More rural, lower academic-level schools were more relaxed.
I guess I haven't helped much other than to say, it depends. Start with a skirt and a top with sleeves, go from there.

now I'm curious about the shoes. They told me in an email to wear shoes for indoors only. I'm guess a short heel that's black?

now I'm curious about the shoes. They told me in an email to wear shoes for indoors only. I'm guess a short heel that's black?

Your indoor shoes can be anything, its just a separate pair of shoes that are only worn indoors (you change your shoes when entering the school building). Pick something that will match the majority of your outfits.

The shoes the other teachers wear might surprise you - many of the men wore what looked like plastic sandals. You'll want something comfortable and low, and it doesn't really matter if they go or not. I wore simple mules or sandals because none of the high schools I went to was (a) adequately heated and (b) air-conditioned at all. I would wear tights with my skirts, to stay warm, though in the teacher's room there was an enormous kerosene heater just by my desk - I couldn't sit there in winter.
The Junior highs I went to, because the funding was different, were air-conditioned. Go figure. They may be getting around to adding a/c to the high schools these days, but still, no central heat. Hallways were FREEZING at my schools.

.... maybe after 18 years as ALT here in the land-of-the-not-always-rising-sun, some hints:

look what the regular female teachers are wearing. It is not as strict any more as it once was. Still, you should not wear tanktops, T-shirts and so on. Shoes, anything is ok, many wear sneekers, as long as it is not toilet slippers you should be good to go. One thing only: don't wear jeans! Normal slacks are ok, of course.