.online_classroom {
    border-collapse: collapse;
    border: 2px solid black;
    font-size: 12px;
}
.online_classroom tr {
    border: 1px solid black;
}
.online_classroom td {
    border: 1px solid black;
    text-align: center;
}
.online_classroom th {
    border: 2px solid black;
    padding: 0;
    margin: 0;
}