Monks should bow to spiritual teachers, regardless of whether they are monastics or laity.
Generally speaking, monks prostrate to buddhas and monks of more senior status. They do not prostrate to monks and laity who are younger or of junior status. Novice monks prostrate to buddhas, fully-ordained monks, and novice monks who are older than them. They do not prostrate to novice monks or laity who are of junior status than them. This is a basic principle in the vinaya scriptures. However, the Prajnaparamita Sutra states that bodhisattvas are above all sravakas, pratyekabuddhas, and sentient beings; therefore, whether they are monastics or laity, as long as they have attained bodhicitta, they are worthy recipients of all sentient beings' prostrations. In other sutras and abhidharmas it states that whether or not they are of senior status, monastic or laity, one should respectfully bow to all one's spiritual teachers. Under certain circumstances, to avoid ridicule from others, one can adjust the method of respect shown to one's spiritual teacher.
The Compilation of Examples From Many Sutras brings up this special case: There was an arhat who had a novice monk as an attendant. One day, the arhat observed that the novice monk was aspiring to develop bodhicitta; therefore, he allowed the novice monk to walk in front of him and carried his bags as a sign of respect for the novice monk. As this novice monk was walking, he thought of how difficult it would be to help all beings and grew discouraged, withdrawing his aspirations to help all beings. The arhat then directed the novice monk to walk behind him. From here, it is obvious that great Buddhist masters believe that anyone with bodhicitta should be treated with great reverence and respect.
When Prostrating to Buddhist Statues, Perceive That One is Prostrating to One's Spiritual Teacher
In The Fifty Verses of Guru Devotion, composed by Ashvaghosha Bodhisattva, as well as The Explanation of the Fifty Verses of Guru Devotion, composed by Je Tsong-kha-pa, both clearly explain whether a monastic student can or cannot make prostrations to his or her spiritual teacher, who is a lay person. To avoid ridicule, the spiritual teacher who is a lay person must respectfully place a statue of buddha in front of him or her before accepting prostrations by the monastic disciple. Though the monastic student is prostrating to a statue of buddha, one must envision that one is actually prostrating to one's spiritual teacher. If there is no need to avoid ridicule, monastic students may prostrate to their spiritual teacher who is a lay person. Like India's Candrakirti Bodhisattva, when he respectfully invited Candragomin, a great Buddhist scholar, to give teachings at Nalanda Monastery, Candragomin humbly replied that he was but a mere lay person and that it was unsuitable for him to accept the monastery's invitation. However, Candrakirti Bodhisattva replied that this would not be a problem and asked the monastics to walk upfront, while carrying a statue of Manjushri Bodhisattva and leading the way for Candragomin, who walked behind them. During the welcoming ceremony, it appeared that they were welcoming Manjushri Bodhisattva, when in fact everyone knew that they were really welcoming the great scholar Candragomin, who was at the very back of the assembly. Thus, this is how the great scholars and Buddhist masters in the past have demonstrated as models of exemplar when they encountered such a situation.