I frequently refer to the Royal Horticultural Society's website for accurate information, and find their School Gardening website particularly useful. They offer free accreditation with their School Gardening Awards (Levels 1-5) and there are lesson plans, information sheets and class growing topics available free of charge. If there was only one website that I could recommend, this would be it: https://schoolgardening.rhs.org.uk
Another site I find of practical benefit is www.thompson-morgan.com They are a private company, obviously aiming to promote and sell their products, but they also provide a 'Gardening Information' section on their website, which has sub-sections such as 'What to do in the garden this month' and 'Gardening Q & A'.
An excellent bug identification guide is available at www.bigbughunt.com It is an international research project, which uses reports from real gardeners to track how bugs and pests spread. Schools or individuals can subscribe to their e-mailing list and receive updates when pests are heading their way. You can report which bugs you've spotted, what plant they were attacking, where and when, thereby helping the project develop an accurate record of pest movement across the UK, Europe and the rest of the World!
With details of bugs, from Aphids to Wireworms, it helps gardeners name those bugs they've seen in their garden, but never known the name of. The site also gives details of the type of plants most affected by each particular species, what part of the world the bug appears in, the type of damage it will cause, and tips on prevention and managing outbreaks.