"Behaving unexpectedly" means:

However, using their school telescope, a team of children and their teacher Jonathan Swift at Thacher School in California have found that more than a month after the collision, Dimorphos' orbit continuously slowed after impact... which is unusual and unexpected.

As reported in the New Scientist, the team presented their findings at a meeting of the American Astronomical Society.

After discovering the unusual behaviour of Dimorphos, it's likely that Nasa will have to factor in the high school's findings, if they ever launch another asteroid redirection mission in the future.