it's necessary that every student ____a uniform

"Get" and "be" in those sentences are examples of subjunctive verb forms.The subjunctive has the base form of the verb in every person, including the third (no -s ending), in both the present and the past. It's used in sentences expressing that it's important, necessary, or recommendable to do something. Thus, it is used in "that" clauses after adjectives like "important" and "necessary" or verbs like "recommend" and "suggest." The subjunctive is mainly used in U.S. English in a formal style. In a less formal style, an infinitive or a gerund ( = -ing form) can be used. Ex.: "It's necessary that you do that." = It's necessary for you to do that. "It was important that you be there." = It was important for you to be there. "I recommended that you go there." = I recommended (you/your) going there.