while($row = $result->fetch_assoc()) {
		$id = $row['idstudent'];
        echo "<tr><td>" . $row["idstudent"]. " </td><td> " . $row["imie"]. " </td><td> " . $row["nazwisko"]. " </td><td> "
		.$row['grupa']. "</td>".'<td><a href="usuwanie.php?action=delete&id='.$id.'">Usuń</td></tr>';
    }


--------------------------------------------------


<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "studenci";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

if ($_POST['action']='delete' && isset($_POST['id'])) {
    $id = (int) $_POST['idstudent'];
$zapytanie = "DELETE FROM student WHERE idstudent = $id";
    $polaczenie->query($zapytanie);
}
$conn->close();
?>