Present perfect continuous in passive voice
Management have been ignoring complaints/suggestions.
If I were your student, I would argue the toss too.
This is clearly an active voice because there is no form of BE+Past participle1 to indicate passive voice. "Been" in this example marks a progressive aspect.
The progressive passive form should be,
Complaints/suggestions have been being ignored by the management.
1This is the typical form of passive. But actually, not all passive constructions are marked by BE because we have bare passive and get-passive. There is also a form of passive that doesn't need a past participle; it's concealed passive.
Your student is right. Management have been ignoring suggestions is in active voice because Management is doing the ignoring. The passive version of this sentence would be Suggestions have been ignored by Management.
The passive forms of perfect progressives are rarely used. The structures of the Present perfect Progressive Active and Passive voices are as follows:
Subject + have/has + been + -ing participle + Object. (Active Voice)
E.g. Management has been ignoring complaints/suggestions.
Subject + have/has + been + being + past participle + (by + Object). (Passive Voice)
E.g. Complaints/suggestions have been being ignored (by management)
More detailed explanations and examples of the Passive voice are available here.