It isn't slang. It is informal, but not slang.
If a phrase or word is slang, it means that it is restricted to certain group of people according to age ( e.g. teenagers) or ethnicity ( e.g. African Americans) or location ( e.g. a certain city or region) or another subgroup ( fishermen or gangsters, for example). The slang of these groups is not generally used outside these groups, and may well not be understood by outsiders.
Everyone knows what 'kids' means, and everybody uses it. You wouldn't use it in every situation - it would be inappropriate in a legal document or business letter and you probably wouldn't used it in a formal essay, either. If that is what you mean by 'officially' , then yes, it should be avoided in some contexts. But it isn't slang. It is simply an informal word. If your teacher said it was slang, he or she doesn't fully understand what the term 'slang' means.