Multiple sources point out that the original rumor of Zendaya playing Michelle Toomes, the daughter of Vulture, may be false. It was revealed late last week that Zendaya would be playing Michelle Toomes from the junior novelization of Spider-Man: Homecoming that leaked early. The junior novel is a very bare-boned version of the story, but it does include the basic plot as well as the names of the characters. This was big news because that would make Michelle the daughter of Michael Keaton's villainous Vulture, which raised all kinds of questions about how that would affect the friendship between Michelle and Peter Parker.
It has now come to light that the junior novelization of Spider-Man: Homecoming does not reveal Michelle's last name. ScreenGeek reports that members of Reddit and Tumblr have the leak of the book in their possession and they can confirm that there is no mention of Michelle's last name in the book. Michelle is simply referred to as "the artsy kid at his school full of science and math whizzes who loves to give Peter a hard time" by author Jim McCann, which basically confirms what we have seen in all promotional material thus far.
So while the rumor has not been completely debunked, it has definitely lost some steam. Just because the junior novelization of the book doesn't mention her last name it does not mean that the rumor is not true. It is quite possible that the last name of Zendaya's Michelle character could be revealed later for a possible plot twist in the sequel. The junior novelization does not even cover the entire movie within its pages and stops just before some major action is set to take place. We'll just have to wait for the movie to come out in less than a month to see for sure.
Prior to becoming the Vulture, Adrian Toomes had a daughter named Valeria with a woman named Cheryl. It is not clear where the rumor started that suggested that Michelle's last name was Toomes, but it seems to have stemmed from an anonymous tip to MoviePilot who provided the embellishment. It's an easy rumor to get wrapped up in because her surname has not been revealed in any of the promotional material released for the new Spider-Man movie, but rest assured, at no point in the junior novelization is her surname revealed.
Again, this rumor has not been entirely ruled out, but it has certainly lost some credibility at this time. It is common practice for junior novelizations of movies to leave out major plot points and this particular book ends before the third act of the movie. It is still possible for Michelle's last name to be Toomes, but it doesn't look like her identity will be a major plot point to Spider-Man: Homecoming. Spider-Man: Homecoming will be released on July 7th, 2017 so hopefully we'll have some concrete evidence after the movie hits theaters.