First of all, do you go to a public school or a Catholic school? I only ask, because it seems to me that a Catholic school ought not host someone who supports such positions. If it’s a public school, I hope they would afford the same opportunity to someone on the other side of the aisle.
All that aside, no it certainly is not a sin to go listen to him speak. You might even wear a pro-life t-shirt, voicing your opposition to his policy positions. They will probably give you time for Q and A after he is done speaking. I would NOT recommend you ask him a question about abortion/pro life/pro family measures. Politicians are professionals at answering questions and I GUARANTEE you will not be able to win an argument with him. What will happen is his position, unreasonable as it is, will appeal to emotion that most young people, sadly, can’t reason past. What will happen is more will agree with him. Nw, if he brings the issue up, that’s a whole different matter. In that case, you might speak up so the pro-life side has a voice.
There are a lot of variables here, but the bottom line is no, it’s not sinful at all to go. You don’t support him. There is nothing wrong with listening to what he has to say. Just don’t get caught up in the lies.