The correct subject pronoun to use in the mentioned situation is:
<u><em>Vosotros</em></u>.
Explanation:
The mentioned case, where a student must address their classmates exposing the different arguments to be selected the class president, refers to speaking for a person (in the plural), which is the first reference for the answer, but also already that <u>the idea is to familiarize with the hearing public and generate certain security towards the speaker</u>, a personal pronoun is not used as <em>"ustedes"</em> that is used with unknown people or with whom you have no confidence but you must use <u><em>"vosotros"</em></u> (who applies to men and women together), <u>which is often used for people with whom you have confidence, which is one of the virtues that you want to radiate</u>.
Explanation: Ellos translates into a masculine or mixed-gendered “They”. Son just means are. Nosotros is out because it’s like a “we” and same goes for vosotros. Es means is more than it means are. Estos means these.