This has been hashed over pretty well but I'm just shocked to read it for the first time. I have all kinds of questions as I find this such as head-shaker but I just don't even want to get into it as you are not someone that I want to put any effort into at all.
There is this old saying about actions speaking louder than words. In this case, your past actions will continue to be much much louder than any words you can say to apologise, or to rationalise or justify. Whichever course you attempt.
I do find it interesting that your boss accused you right after he found out about it, meaning you were likely already a disruptive or negative influence. He wouldn't accuse a quality or star employee so it wasn't just the prank, there was other issues here as well.
Your claims of not knowing the seriousness of your actions only continues to show an incredible lack of maturity and judgement. I agree with Sergei, you are dangerous as you were messing with someones professional reputation and likely also the company image. These are lines you DO NOT CROSS if you wish to work for another person or company. Even a child should not need to be told this!
So I suggest you accept that you got off easy and move along, and likely away!
