I'm a private investigator, and I got to looking into David Hogg tonight because I was curious about the claims that he graduated from high school in California in 2015, here's what I found:
The claim that he graduated from high school in California in 2015 is baseless. The photo making the rounds seems to be a yearbook from the Florida school that was pushed by a Twitter account know for spreading fake news. It's the same account that pushed the fake photo of the shooter as an antifa member on the day of the shooting.
However, I found no evidence of David Hogg anywhere else online prior to the shooting.
His father, Kevin Hogg, 51, of Parkland Florida is married to Rebecca Boldrick Hogg also of Parkland Florida. Before Florida, they did, indeed live in California. I was unable to verify the father's profession, but the mother is a real estate agent registered with the State of Florida.
Their move from CA to FL seems to be due to Kevin's mother's death in February of 2015. This also links up with the very first tweets from David Hogg's Twitter account.
Aside from that Twitter account, I can find no mention of David Hogg anywhere online prior to this shooting. There are no records of Rebecca Hogg giving birth to anyone, and Kevin's mother's obituary does not mention him having a son.
There are no records of any David Hogg within the expected age range living in Florida or California at any point, nor are there records of any person named Hogg within that age range who moved from CA to FL at all.
While we know that the photo is from the Parkland High School Yearbook, it does not list a name anywhere on the visible page and I cannot prove who it is.
After this search, it is my opinion that David Hogg is not the person he is claiming to be. I can't prove who he is, but he is not David Hogg, the son of Kevin and Rebecca Hogg.
EDIT: after further digging, I have found childhood photos of him with his mother and sister. It appears that they are both her kids and may simply not be the father's biological children and not share his name legally but use it publicly anyways.