Among other responsibilities, the U.S. Secret Service is directly charged with keeping the president, his wife, and their children safe. Each member of the presidential family gets 24-hour protection from one or more Secret Service agent(s). As Malia and Sasha Obama grow through adolescence during their dad's stint as President, of course they'll wish they had some privacy. They'll want to hang out with friends, go to school dances or events, and maybe even have private phone conversations with future boyfriends. How will they handle the constant surveillance? (How would you like it if there was a grownup holding onto the back of your pants every time you walked through a crowd, or listening to your every phone call from a secret extension?)
For one thing, the Obama girls have had some time to get used to their new bodyguards. President Obama, Michelle, Sasha, and Malia have all been under Secret Service protection since the middle of 2007 because of the presidential campaign. Secondly, the Obama girls are attending a school that serves many families of diplomats and government officials — who are used to this kind of thing. Maybe you can't imagine a dude muttering code words into a shirt-collar microphone in your classroom, but this is everyday life for the kids in some Washington DC schools.
The movies often show Secret Service agents in dark suits and sunglasses, but in real life, the agents try to blend in. Agents who are with the President when he's on official business wear suits; in school with Sasha and Malia, they're more likely to be in polo shirts and khakis. If they go to a school dance, they would be in formal wear and may not look any different than any other teacher or chaperone; if they go to a game, they might be in jeans and a school sweatshirt and look just like any parent.
In the classrooms, Sasha and Malia's Secret Service protection will be present but most likely not hovering. They would have run background checks on any teachers (and maybe students) that come in contact with the president's daughters, so there's no reason for the agents to be attached to the girls at the hip. Instead, they might set up a command center in the school and tap into the school's closed-circuit camera system. When they are in classrooms, they might act more like teacher's aides most of the time — teacher's aides with guns, yes, but hopefully they'll never have to use them.
Over the years, the Secret Service has learned to protect the First Family while also affording them at least an illusion of privacy. Both Amy Carter and Chelsea Clinton were able lead fairly normal lives during their fathers' presidencies. Amy even attended public school in Washington, while Chelsea was active in ballet and with her church youth group. I'm sure the Obama girls will feel the same.