For anyone learning to play an instrument who wishes to take ABRSM Grade 6 or above, a certificate in Grade 5 Theory is a requirement (although Jazz Piano and Practical Musicianship are offered as alternatives - please ask if you are interested in either of these).
Music theory also provides essential knowledge for those considering music at AS and A'Level, or who wish to study music at higher education.
For these reasons, music theory is a built-in feature of good music teaching from the earliest levels of playing. However, once pupils reach around Grade 2 piano it becomes increasingly important for them to study music theory in more detail.
The importance of fitting music theory into lessons alongside practical music making is one reason that all students are recommended 45 minute lessons, other than the youngest beginners.