This is the 4439th most frequent Russian word.
внимательными
"Attentive" or "careful."
The word 'внимательными' is used here as an instrumental plural adjective modifying the implied noun 'students', describing the state or condition they should maintain.
Учитель просил нас быть внимательными на уроке.
The teacher asked us to be attentive during the lesson.
The word 'внимательными' functions in this sentence as the instrumental plural adjective, describing the subject's quality during the action.
Они были внимательными, слушая доклад.
They were attentive while listening to the report.
Here, 'внимательными' conveys a persistent characteristic or state, used in instrumental plural form to describe how children behave.