I do agree that there are ways to implement this incorrectly, and my support is contingent on the following:
* Working with minorities in a field does not mean refusing to work with the majority. To ground this in the world of tech, reaching out to girls and underrepresented minorities does not mean turning away Asian & white boys.
* Steering them towards STEM should leave the agency in their hands. Even in the absence of all social and cultural pressure, it's a stretch to assume that we'll have gender parity in all fields. I'm confident in saying that software development(I specifically point out software development since some STEM fields have parity or even a slight majority representation of women) would have a greater representation than it does now, but I don't think it's 50%. If diversity initiatives end up pressuring people into fields they don't want to be in, then that's a failure in and of itself.
Working with a specific class of students (based on gender/race) is highly discriminatory, IMO. I think it is unfair to the people not included in the group you want to encourage, specially when it is children we're talking about. Why not have STEM grooming programs for any student who wants to join? Why exclude some? I live in India. We have a history of casteism. Measures like this were instituted 70 years ago to help the downtrodden. Now it prevents people like me from getting a place in the country's most prestigious colleges while other less-deserving people are admitted because of their lower caste. I was born in the 90s. I did not even once witness an incident of casteism in my life. Except the one perpetrated against me. Your suggestion seems to me like a similar measure. It isn't much different from a quota system.