There were real hippie movements that mixed it with spiritual stuff that only appeals to a minority. Hence some labelling by association.
(I dont feel very motivated to work in blockchain -- because by association it carries elements of crypto hype that I find distasteful)
Once you separate the physical excercise and its benefits and allow people the space to pursue them for their benefits without necessarily subscribing to new spirituality -- it has found broader appeal.
(Few decades ago smoking was widely accepted socially even though it was no secret that it damages health. It took a few decades of mounting scientific evidence for society to shun smoking. Takes time for society and cultures to accept new ideas.)
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Resistance to teaching in schools ... from who?
Schools in india have a lot of shortcomings when it comes to physical education, sports and other life skills training -- yoga / meditation alone does not get any special treatment either for or against.
There is a lot more our schools can do better -- including surely physical exercise and meditation/breathing exercises.
If anything there is too much emphasis on making international yoga day a grand spectacle one day each year -- which surely involves involuntary participation by schoolkids -- with no lasting benefits except PR.
Even the most vocal debates of today about school curriculum (see CBSE / NCERT in current affairs) dont have any arguments for or against extra curricular or non academic skills in school.
We broadly have a long way to go in making our education curricula well rounded. Would welcome more genuine elements of physical wellbeing and meditation included (without any burden of spitiruality mixed in -- which can create the resistance you might be referring to).
There is still resistance in India to teaching this in government schools.