Hi,seems you and I are very much in the same boat haha! My first piggy Vinny was alone her whole life which I deeply regret but I was stupid and did little research. School obviously means I couldn't spend 24/7 with her and sadly that's not good enough. If I could turn back the clock I would definitely have got her a friend.
My newest piggy (Vinny was put to sleep in 2015 after turning 3 for meningitis) was bought in Feb 2016. I got him by himself as mum finally agreed I could have another-She didn't want me getting attached like I did if they were only gonna get sick again thankfully what Vinny had is rare. Anyway I was aloud Mo and since that day I've been showing mum how he should have a friend. She's said yes now (just my stepdad to go) but Mo couldn't have a friend earlier on anyway because he had a URI scare and we were advised to wait and see how we went.
In my case,my friends sows that she rescued have had a litter of 3 pups each and she has offered to give me a boar to try with mo. So at least if it didn't work out the baby would just go back home with her. I know it can be frustrating but don't constantly bug your parents. Understand their points and do your own research/digging on ways around them. If they have allergies- is there something you can do when caring for your piggies that could make them a little better? Explain about how guinea pigs need friends and show them how responsible you can be.
Hope this helps and good luck.