"Boys don't go to girls' initiation"
Translation:Wavulana hawaendi kwenye unyago
Unfortunately, the -enye construction is not taught in this course, although it seems to be fairly common. Some examples that I found:
Mwanaume mwenye gari: a man who has a car, a man with a car.
Wanaume wenye magari: Men who have cars, men with cars.
Kitabu chenye picha: a book with pictures
Vitabu vyenye picha: books with pictures
etc, etc. So, kwenye unyago translates to 'a/the place with unyago'. More about Unyago on Wikipedia: https://en.wikipedia.org/wiki/Unyago