When your students log into their Sumdog account (unless restricted to work on Tables) the default activity will be Training.
The students will be able to select Tables from their Activity panel on their City page.
They will then be able to select if they would like to work on:
If any of these options are greyed out it means that another teacher in the school has disabled the tables option from the School details page. Scroll down to Activities and make sure that there is a green tick next to all the tables activities.