When students become upset it may seem easier to simply send them out of the classroom and resume the lesson, this strategy has many long term negative consequences for the individual student. First, the student misses out on an opportunity to learn and practice de-escalating their emotions. Secondly, the student learns that their emotions are something which they must deal with alone. Students also come to learn, even if unintentionally, that they can be punished for their emotions.
This Edutopia article provides a step-by-step de-escalation technique for teachers to use with upset students. https://www.edutopia.org/article/de-escalation-exercise-upset-students