JSFiddle - React, Tailwind, and code Playground

HTML

<div class="group_name">Group A</div>
<div class="student_name">Student 1</div>
<div class="student_name">Student 2</div>
<div class="student_name">Student 3</div>

<div class="group_name">Group B</div>
<div class="student_name">Student 1</div>
<div class="student_name">Student 2</div>
<div class="student_name">Student 3</div>

<div class="group_name">Group C</div>
<div class="student_name">Student 1</div>
<div class="student_name">Student 2</div>
<div class="student_name">Student 3</div>

<div class="group_name">Group Z</div>
<div class="student_name">Student 1</div>
<div class="student_name">Student 2</div>
<div class="student_name">Student 3</div>

JavaScript

$.each(['Group A', 'Group B', 'Group C'], function(i, val) {

$('.group_name').filter(function() {
    return $(this).text() == Group A;
}).nextUntil('.group_name').filter('.student_name').last().after('<div class="student_name">Student X'+i+'</div>');
    
});