The W Tutors system does not allow 1/2 hour lessons except for trials. All lessons are booked in blocks of one hour
Nevertheless, you may conduct 0.5 or 1.5 or even 2.5 hour lessons through the online lesson space. Once you and your learner have entered the lesson, you may use it for as long as you would like.
You may also adjust the rates to charge your learner for the extra/reduced time. Simply go to "My Learners" from the dashboard and toggle the lock icon beside his or her name to enter new prices just for them. More information is available here