Two briefs, one invitation
The teenager's requirements are short. No baby photographs, no exclamation marks, no cartoon, and absolutely no chance that a screenshot of this goes round the year group with a caption. Your requirements are also short: whether there will be twenty or fifty people in the house, who is driving them home, and whether the food you bought is enough.
These do not conflict. A clean page with real information reads well to a fourteen-year-old and answers to a parent, which is more than a poster in the class group manages. The trick is to let the design be quiet and let the details do the talking.