Actually, the crb check may not be a requirement if you’re only giving talks to groups and will never be left alone with them. Talk to a friendly school that you perhaps already have a link with and let them be your guide.
(I work in schools all the time. I am crb checked because I regularly work one to one, but for schools where I am visiting just to do a group workshop it hasn’t been a requirement. The rules keep changing on where a crb check is valid depending on who applied for it. Years back, if you had a crb check it could be relevant at other establishments too. Then it went through a period where every different establishment had to apply for a crb check for you, regardless of whether you already had one still in date. Now it’s returned to the more common sense approach. It’s a long-ish process waiting on paperwork, and it costs the school money, so they are unlikely to do one for a one-off visit.)