@Puffalicious welcome back.
Great advice already from @TheRealHousewife
My 13 year old is also ND and I also teach. DC started secondary school this year and after settling well the last few years in their Special Ed Primary, secondary has been a big transition and I'm having to support a lot at home with academics (so many tests in the system I'm in, even special ed). My youngest is about to be referred for diagnostic process so the whole merry go round will start again. DP does what he can but struggles with own MH (likely ND) so a lot falls on me. Have no family support as I'm not in the UK with my own family and DP's family are somewhat fractured - estrangement, elderly ILs who have never been proactive in helping with DC much. So, I know how it feels when you're struggling to exercise.
I find keeping it simple is the best. Mostly I either walk (during school holidays I make DC come with me, or I escape after youngest DC bedtime for a walk around the neighbourhood - can only do that as I'm urban and not rural. Sometimes I go during a free period at school or on my days off after the school run) or do yoga.
I did a lot of yoga this year - from May to October I tried to do yoga early in the morning, outside before kids up (first year I've been able to do it as teen is not as likely to get up as soon as I stir). I always do short practices (less than 30 minutes) and sometimes this means making sure the kids know I shouldn't be disturbed for a short period of time(in practice, someone always starts fighting so if I can do this when they are out or asleep it's better!). I like Yoga with Adriene on YouTube.
Also anything is better than nothing. Dr Chatterjee has a kitchen workout in one of his books that he does while his coffee is brewing (5-10x calf raises, squats, lunges on each leg, tricep dips, push-ups) and if you do it daily it will really make a difference (If I manage it I only do 5 - the push-ups finish me off). Or find a 5 - 15 minute quick workout that doesn't take lots of space. There's tonnes on YouTube that don't need equipment or space (try Jessica Smith - you don't even need to get changed!).
I used to be a runner pre-pandemic, but haven't done a race for 5 years now. I may retry couch to 5k but get frustrated with all the niggles that seems to have appeared the last few years (Find it annoying that I could run 10-15k no problem and now even a 5k is out of reach).
Pick whatever works for your own mental health, For me being outside is really important (I think that's why I've struggled with yoga in November and December as I hate practising inside in my messy cluttered house).
I think it's great that you haven't weighed yourself. Make your focus on feeling good about yourself. Find the habits that make you feel healthy. I know that eating too much bread makes me feel bloated and sluggish after lunch, as does eating anything sugary. So I stick to fruit/veg/unsalted nuts for snacks (wherever possible). I know if I don't drink enough water I get headaches, constipation and am more likely to eat rubbish. I know that eating anything in the evening after dinner (but especially cheese, crisps) will give me a bad night's sleep, dry mouth and headache in the morning. Alcohol doesn't agree with me and affects my sleep. If I don't go outside and move I will be in a bad mood.
Get yourself a haircut and look after it (condition, avoid the hairdryer or make sure it's protected from heat, use hair clips instead of ties to avoid damage), look after your skin (just the basics is enough! cleansing and moisturising twice a day), get enough sleep, pick up a few new clothes that you feel fabulous in (I've just started following https://www.instagram.com/imlaurabradshaw/ for comfy inspiration).
Don't try and do everything at once, pick one thing you want to do and give it a try.