There is now more of an emphasis on grammar in the primary school national curriculum in England and Wales (Scotland and Northern Ireland have their own system, which I’m less familiar with), and this has prompted publishers and online sites to come up with exercises to practise specific grammatical points. Indeed, I have been asked to write several such exercises myself. However, it is not an easy task, mainly because so many words in English do not fit neatly into a grammatical category.
Take the word worth for instance. Most dictionaries call it an adjective (eg OED, ODE, Chambers). However, it doesn’t behave like a typical adjective. You can’t say “this book is very worth”, or “this book is worther than that one”. It needs to be followed by something, eg this book is worth £10/ a lot/ buying. For this reason, other sources eg Longman Online Dictionary, describe worth as a preposition. Some books and sites (eg a UCL grammar site) call it a marginal preposition, as it has affinities with other word classes (other examples of marginal prepositions are plus, minus, following, and concerning). Collins Dictionary says of worth that it is an adjective but one that governs a noun with prepositional force.
Whatever class of word the word worth is, it is still not suitable to include in a multiple choice exercise for children where they are asked to spot the adjective, or spot the odd-one-out (and there are many other such confusing words in English).