First, thirty years have passed since then. Marty was in 1955 for only a week, and in that week he was actually with George and/or Lorraine for a matter of hours. Yes, he had a big influence on their lives, but all the same they did not see him very much, and it's not likely that they would remember his face after thirty years. Many of us have trouble seeing the faces of people we've seen hours ago. Second, it's not as if they suddenly see Marty again out of nowhere. He is their son; they had brought him up from birth, seeing him grow up through the ages, and by the time he gets to his teenage years, their memories of Marty's exact appearance in 1955 have faded. Even if they did think there was a resemblance, wouldn't it be likely that George and Lorraine would either a) put it down to a coincidence, e.g. Lorraine: "Hey George, you know who Marty reminds me of? That guy Calvin Klein we knew for a week in High School!" George: "Yeah, I remember him. Ha! That's kind of funny since that's where we came up with the name Marty, huh?" OR b) consider it to be the same guy? Lorraine: "George, I can't believe this, but I think our son Marty is the same guy we knew for a week in 1955!" It's also unlikely George would have ever being suspicious of Marty being anyone else's baby because he would have resembled George from birth, and the way he is as we see him at 17, possibly has similar features of other relatives like grandparents etc when they were younger who George and Lorraine are used to seeing in old family photos. Of course, it not mentioned whether George ever noticed the suspiciously close resemblance between the boy he thinks is his son and his wife's old boyfriend, or if he ever broached the subject with Lorraine.
The other theory is the parents after Marty returns back from 1955 do have a slight recognition of him. Keep in mind after the dance in 1955, Lorraine comments on Marty having a nice name. This in retrospect could have resulted in his name being Marty out of respect due to them being together. He is given the name in both timelines, however is treated differently in both. In the first timeline, Marty is detested by his mother from dating and not much help from his father on the subject. In the second timeline, his mother is very open to him dating and is given his dream truck by his parents as well. His parents having the recognition of him being the same buy is highly unlikely, but the effect he had on them in 1955 did alter their relationship on the Marty in the new timeline.