The lack of high-level assistive control strategies in the field of teleoperated surgical robots has been linked to intra-operative injuries and increased fatigue experienced by practitioners. Virtual Fixtures analogous to the one proposed in this study may be beneficial for the patient's safety and the outcome of the operation; this work aims at evaluating their effectiveness in the context of surgical training. Tracking the position and orientation of a teleoperated surgical instrument with respect to a reference trajectory - planned in the pre-operative phase - allows one to compute and apply feedback forces to the manipulators held by the practitioner, which will provide haptic guidance towards an improved surgical performance. This high-level control strategy is here tested on a suturing task emulated in a virtual environment, where a group of participants was evaluated on the distance and angle error committed during the execution with and without assistance. The assistive modality proposed here is able to reduce the average error committed in the execution of the virtual suturing task: virtual fixtures and other similar assistance mechanisms may be most beneficial in the surgical training scenario, improving the learning curve and achieving better performances.