I would start by contacting the IEEE professional ethics team (or ask for the most senior director related to that area you can get through to). State when they ask who you want, "I'm not sure: it's a serious issue of professional ethics, I really need to speak to a director or someone that sort of level, or their PA, someone very senior, not an ordinary customer facing person or service staff."
You'll be asked your name/number - perhaps told they can't put you through without it. State that you want this on a "no names" basis, and please just find such a person and put me through. Be polite but firm, and expect a long time on hold, this won't be usual and they will have to check what to do and who to put you through to.
If the person is in fact senior, you can add that this is because you are fearful of repercussions. In my experience people will very easily agree to this - if not it's the wrong person. Getting put through to the right person is usually the hardest step, if needed Google and figure who you want to talk to (and direct phone?) and then be prepared to push hard. Example: "Something's happened which I need to discuss with the director responsible for ethics. Can you put me through please". Then "sorry, I need the director responsible for ethics. As you can understand, I don't feel the matter can be told to other people for passing on, or I'd have left it with the customer services team" followed by "can I speak with the manager in charge" or "please just find who I need to speak to and put me through". Be prepared for a few conversations like this, repeated escalation, and understand it's unusual for them, too.
A good trick is to ask for the person's PA, instead - PAs are usually very good for this sort of thing, and if you get to them, you can be more open, and they are easier to get to as well.
Eventually, assuming you get through to someone who sounds senior and willing to talk/listen, tell them the situation without names or institution/university details, and tell them you feel they should know but fear consequences, and this is far beyond what you are used to. You don't know whether or not the IEEE rules forbid or allow it even. But you feel ethically bound to report it. You don't however want to formally report it as you would have to provide names, dates, testimony, or proof (voice recording etc?). So you are in a quandary. Can he/she advise.
Then see what they say. They will surely want to know more, which you can't tell them, so ask them what else can be done. Can they consider it and call you back (friends phone) or email you (use a throwaway email address). That might be best.
In my experience directors etc often care passionately and want to hear if something's wrong enough to merit their attention - and forcing students' votes to manipulate the election almost certainly would be.
But if at all possible get proof.
You don't say what country you are in, and IEEE is worldwide. If legal in your country (and allowing for legal exceptions related to professional misconduct/coercion/abuse of position of power by professionals, as some countries consider this to be criminal misconduct not just "bad conduct"), you may also be legally allowed to take a voice recording app and a phone loosely held in the hand (just saying " 'bye" when he/she opens the door as if it's end of a call may also help) when you ask your supervisor is all it takes: "please, I'm not really sure it's okay to tell me who to vote for, in the IEEE, can you retract the direction, I feel very conflicted and stressed over it" or "would it be okay to vote privately"?, and try to capture them saying you have to do as told (if they do).
