Подсветка первых 5 мест в таблице

Лучше это сделать без присвоении лишних классов

table.tournament tr:nth-child(-n+5){ 
	background: yellow; 
}

<table class="tournament"> 
		<tr> 
			<td>1</td> 
			<td>2</td> 
			<td>3</td> 
			<td>4</td> 
		</tr> 
		<tr> 
			<td>1</td> 
			<td>2</td> 
			<td>3</td> 
			<td>4</td> 
		</tr> 
		<tr> 
			<td>1</td> 
			<td>2</td> 
			<td>3</td> 
			<td>4</td> 
		</tr> 
		<tr> 
			<td>1</td> 
			<td>2</td> 
			<td>3</td> 
			<td>4</td> 
		</tr> 
		<tr> 
			<td>1</td> 
			<td>2</td> 
			<td>3</td> 
			<td>4</td> 
		</tr> 
		<tr> 
			<td>1</td> 
			<td>2</td> 
			<td>3</td> 
			<td>4</td> 
		</tr> 
		<tr> 
			<td>1</td> 
			<td>2</td> 
			<td>3</td> 
			<td>4</td> 
		</tr> 
		<tr> 
			<td>1</td> 
			<td>2</td> 
			<td>3</td> 
			<td>4</td> 
		</tr> 
	</table>

Для подсветки строк все зависимости от сортировки, нужно добавить класс строки таблицы:

echo '<table class="tournament" border="3"  >';
echo '<thead>'
echo '<tr>';
echo '<th></th>';
echo '<th class="main_name" >Team name</th>';
echo '<th>Matches Played</th>';
echo '<th>Matches Won</th>';
echo '<th>Matches Won OT</th>';
echo '<th>Matches Lost OT</th>';
echo '<th>Matches Lost</th>';
echo '<th>Goals Scored</th>';
echo '<th>Goals Missed</th>';
echo '<th class="team_points" >Team Points</th>';
echo '</tr>'; 
echo '</thead>';
echo '<tbody>';
$i = 1;
while ($row = mysqli_fetch_array($data)) { 
    // Display the score data
    if($i<=5){
        echo '<tr class="marked">';
    }else{
        echo '<tr>';
    }
    $i++;
    if (is_file(GW_UPLOADPATH . $row['team_logo']) && filesize(GW_UPLOADPATH . $row['team_logo']) > 0) {
        echo '<td><img class="playerphoto" src="' . GW_UPLOADPATH . $row['team_logo'] . '" alt="Photo" /></td>';
    }else{
        echo '<td><img src="' . GW_UPLOADPATH . '' . '" alt="" /></td>';
    }
    echo '<td  class="team_name" >' . $row['team_name'] . '</td>';
    echo '<td> ' . $row['matches_played'] . '</td>';
    echo '<td>' . $row['matches_won'] . '</td>';
    echo '<td>' . $row['matches_won_ot'] . '</td>';
    echo '<td>' . $row['matches_lost_ot'] . '</td>';
    echo '<td>' . $row['matches_lost'] . '</td>';
    echo '<td>' . $row['goals_scored'] . '</td>';
    echo '<td>' . $row['goals_missed'] . '</td>';
    echo '<td>' . $row['team_points'] . '</td>';
}
echo '</tr>';
echo '</tbody>';
echo '</table>';

В css уже прописывать стиль:

table.tournament tr.marked{
    // стиль
}