JSFiddle - React, Tailwind, and code Playground

JavaScript

// Функция-фильтр
var filterStudents = function (students, group) {
    return students.filter(function(student) {
        return (student.group == group);
    });
};

// Использование
var groupmates = [
    {
      "name": "Andrey",
      "group": "912-2",
      "age": 19,
      "marks": [4, 3, 5, 5, 4]
		},
    {
      "name": "Artem",
      "group": "912-1",
      "age": 18,
      "marks": [3, 2, 3, 4, 3]
    },
    {
      "name": "Lisa",
      "group": "912-2",
      "age": 19,
      "marks": [3, 5, 4, 3, 5]
    },
    {
      "name": "Irina",
      "group": "912-1",
      "age": 18,
      "marks": [5, 5, 5, 4, 5]
    }
];
console.dir(filterStudents(groupmates, '912-1'));