"booed up" comes from African-American Vernacular English or Multicultural London English and means being in a romantic relationship; emotionally involved with someone. It has spread well beyond its origin through hip-hop, drill, TikTok and gaming, so a teen of any background may use it. It is not in itself a sign of bad behaviour; it is a style of speech the way 'cool' or 'wicked' once was. The conversation worth having is about respecting where the language comes from, not about banning the word.