I'm not from the US, is the American education system really like High School Musical shows it?
I've never seen High School Musical, but based on the plot I'm going to say no, that's not realistic. At least the part where high school is the entire universe and everything you do in your day to day life is somehow connected to it. Most high schoolers just want to get the hell out at the end of the day and don't have any interest in extracurricular drama. However, having your entire teenage life revolve around high school was very common back in the late 60s to mid 90s, which is when everyone involved in making that movie went to high school.
Depending on where you are, there are four major types of public, non-charter school. Please keep in mind that none of this is a joke or an exaggeration, even though it sounds like it.
Type 1: Inner city type. Populated mostly by Disenfranchised And Oppressed Brown BIPOCs Of Color. The teachers don't even try. As long as nobody is stabbing each other, students are free to do whatever they want, which almost always means either not showing up at all or dancing to loud music during "class". The majority who graduate can barely read at an adult level, much less do math. If anyone points this out or attempts to improve the situation, they are cancelled for racism, because obviously the only reason the children are illiterate gang members is because white supremacy made them do it.
Type 2: Rural type. Students don't take it seriously but at least they show up. The atmosphere is cordial because everyone's families know each other. Middling to average overall education. Demographics match the surrounding area, which depending on location ranges from very white to very Latino. Those who graduate will be able to get a decent job and contribute to society, but don't expect excellence except by chance. Half of the girls will be mothers before graduation. If you're male and don't like sports you're "that weird kid who might be gay", but nobody will bully you for it unless you're in an extremely hickish area. Near 100% chance that the military will try to recruit you if you go here.
Type 3: Woke type. Overall quality of education in the hard sciences is good. All other classes are less than useless without a very good anti-wokeness filter. Almost all kids are white except for the token black kid in each class. Teachers will actively encourage anti-white racism. Expect at least one kid to kill themselves per year, after which everyone will pretend they have no idea how this could have possibly happened even though everyone knows exactly how. Teachers and especially administrators get paid suspiciously well. Can produce a good overall education if the parents are extremely aware of what is being taught and are constantly running interference to combat propaganda.
Type 4: Prestige type. Each state has a small handful, sometimes fewer. Teachers and administrators actually care, or at least pretend to in order to keep their jobs. Attended and run almost entirely by an equal mix of whites and Asians. Very tightly run ship, in part because of constant input from helicopter parents. Zero tolerance for bad behavior. Many parents will drive their kids half an hour or more every day to get them in this school. There are a fair number of geniuses, sociopaths, and sociopathic geniuses. Unless you have connections, are a non-Asian minority with good grades, or get extremely lucky, you do not go to this school.