"Les gens" : people. "Les gens" is spoken French, don't use it in written French. No singular form. Les élèves : pupils. (Les étudiants : students). "Les personnes" : people too, but more polite than "gens". Can be used for one or many individuals. You'll often hear : "la personne suivante, s'il vous plaît". Or "les personnes ayant réservé par téléphone sont priées de se présenter à l'accueil".

les gens = people in general; it represents a multitude of groups of people. Also the word "gens" can only be used in plural les élèves = students les personnes = people; it can be used in singular and plural; it defines a group of individuals from a larger set, which is why we often use the word "personne" with an adjective to further define that individual.