Prayers for you Bro. Harassment and getting made fun of really sucks. Sadly, it seems like it doesn't end when you finish school. It's one of those things that you'll find everywhere for the rest of your like - at your job, from family members, friends, online, ect. Online it's easier to Mute someone or just Leave the Server. But when you're on a Job that you're forced to stand by them for hours at a time - well, it becomes more necessary to find a way to cope to make it through the day.
First, I think asking God's Advice is the Best avenue! He's the Only One who knows what would be best in each unique situation. In your prayer time ask Him to guide your response in this situation and to open the doors to talk to who you need to to help resolve this. Looking into the Bible sometimes you can find an example of how someone else dealt with a similar situation. The Bible explains to us that there is nothing new under the sun.
Ecclesiastes 1:9
What has been will be again, what has been done
will be done again; there is nothing new under the sun.
Things like famine, strife, hatred, war, envy, harassment , people taking advantage of others, physical abuse, lying, cheating, greed, and much more... are things that are (sadly) nothing new. So even though the Bible was written long ago, it holds valuable advice on dealing with situations that keep happening to people all over the world again and again.
Secondly, I think asking the Advice of God loving Family, Friends and Peers is awesome! So you're Right on Track for making a good choice with how to deal with this. I applaud you!
Humor...
In my life, I've found that joking around helps me deal with adversity. When people say things to me that are hurtful or not true I'll respond with something totally silly, maybe sarcastic, stupid or off the wall. Humor is always a touchy subject and what one person finds silly another will find offensive so sometimes it's hard to walk that line. For me and example would be...
You're a Pot Head and you abuse you Girlfriend.
My response might be...
Well, you should know I buy from your Grandma! And she says I still treat her better than you do!
And then laugh it off like you're just joking around with them. I'm just messing with you. Once again humor like that is totally subjective and it might catch them off guard enough to get them to back off or even laugh. For one thing, it lets them know that you're not taking their claims about you seriously. For another thing it will let them know that you're not planning on giving them the reaction they want from you - getting all mad and worked up.
Ignore...
You may just choose to ignore what people are saying about you. You know it's not true so you have nothing to worry about. There will always be people saying something about someone. Sometimes they just do it out of boredom and wanting to start some controversy for excitement. If they don't get the reaction they want it may just die because it's not fun for them any more.
Olive Branch...
You might choose another approach and offer them an Olive Branch of Friendship. You might say to them... You know, you and I don't really hang out at all. Maybe we should do something together like pizza or a game night or a movie sometime. I could even invite my girlfriend along and you could bring a friend too. Maybe by spending some time with you and your girlfriend, they might see that what they are saying about you isn't accurate and it may just open up a possible friendship.
Straight Honest Feelings Approach...
Another approach, you might just want to come right out and tell them how you're feeling. You might say... Ya, I've been having a rough time lately but doing pot would never make things better. It doesn't solve anything. And sometimes my girlfriend and I have our share of disagreements but for me Abuse is never an option. I care about her too much.
*Just remember what God tells us in
Matthew 10:16
I'm sending you out as sheep in the midst of wolves,
so be wise as serpents and innocent as doves.
Be aware of situations and people and the feelings they are having, but try not to just lash out at them in revenge or to hurt them back. The World wages war like this - but the Bible encourages us to be wise but gentle. In your response to hatred and anger, make sure you're standing innocent before God and not sinning back at them in anger.