Although ART would expect your mentor to be on hand to support you for some of your practices/workshops, they don’t need to attend every single one.
You might find it easier to keep in touch via email or phone – send them your practice plan beforehand, explaining the exercises or methods that you’re going to ring and why. Then write up a little report – how it went, what you’d do differently next time and what worked well.
You can identify areas where a particular ringer is struggling with something and discuss this with your mentor, ask for feedback and suggestions if you’re stuck. Your mentor should be there as a supportive friend, as well as offering you constructive feedback.