The instruction arrives every year in some form: embed sustainability across the curriculum. The training budget attached to it is usually nothing, and the free courses that surface first in a search are a mixture of genuinely good work and marketing with a certificate stapled to it. I get asked to sort one from the other often enough that it is worth writing down.
One disclosure before the list. I sell school clothing and I run workshops in schools, so I have an interest here. The list below is things I did not make and do not profit from, and I have put what I do at the end where you can ignore it.
What I would actually spend an evening on
WWF UK runs free certificated professional development for teachers, including its Education for a Sustainable Planet course and its Sustainable Futures Programme. It is the first thing I point people to, because the pedagogy is written by people who work with schools rather than by a communications team, and because a certificate at the end matters when you have to evidence CPD to a line manager.
After that, two categories rather than two names. Your own subject association is almost always the best source for anything discipline-specific, because a geography teacher and a design technology teacher need very different things from the same topic and generic courses serve neither well. And UNESCO publishes free guidance on greening the curriculum which is not a course at all, it is a document, and it is more useful than several courses I have sat through. Eco-Schools materials are worth having open alongside if you are working toward accreditation, because they tell you what evidence looks like.
I would treat anything hosted by a brand with a product in the space, including anything hosted by me, as a resource rather than as training. That is not an accusation. It is just that the incentives are different and you should know which you are reading.
What online modules are good at
They are good at the systems level. Carbon, water, energy, the shape of the problem, the vocabulary, the confidence to stand in front of a class and not be caught out on a definition. They are good at giving you a defensible position when a parent challenges something. They are efficient, they fit around a Tuesday evening, and they can be paused when a child needs something.
What they are not good at
They leave you with knowledge and no object. Every teacher I have spoken to who did a good online course and then tried to teach from it hit the same wall: the material was general and the classroom needed something specific that a fourteen-year-old could hold. Knowing that the textile industry has a water problem does not tell you what to do on Thursday period four. That is not a failing of the course. It is a limit of the format.
So pair it. Whatever you study online, attach one physical thing to it before you teach it. Weigh the lost property. Read the composition labels in your own class's polo shirts and build the numbers from those. Write to your uniform supplier with three specific questions and read the answers aloud, including the ones that do not really answer. The theory gives you the frame and the object gives you the lesson, and in my experience the pupils remember the object.
The bit where I declare my interest properly
I run awareness talks and workshops in schools, in English and in Spanish, built around exactly that pairing. I started doing it because my son reacted to his own school uniform and I discovered how hard it was to find out what he had been wearing, and I have kept doing it because a class holding a garment and asking where it came from gets somewhere in fifty minutes that I could not reach in a term of slides. If that is useful to you, ask. If it is not, the list above still stands on its own, which is rather the point of putting it first.