Poor parents. They only just figured out what in the world Snapchat is and along comes Musical.ly.

The app you've probably never heard of has been banned for those under 13 in an Australian school thanks to privacy issues.

The all-girls school Wenona in Sydney raised concerns about Musical.ly on campus amid fears primary school students were copying "highly sexualised dance moves" seen on the lip-synching app, Fairfax Mediareported. According to a letter sent from the school to parents Monday, girls were also uploading clips while in school uniform, making them highly identifiable.

The first thing to know about Musical.ly is that it's incredibly popular, winning the same breathy coverage as Snapchat in its early days. It's among the top 40 most popular apps in Apple's App Store, and claims to have more than 100 million active users. Interestingly, although Musical.ly seems as American as apple pie, it's actually Chinese.

If you're starting to feel impossibly old, here's how it works: Using 15-second clips of popular songs, users, or "musers," can create mini-music videos and share them on the platform or on other social media apps.

Watch a few and you get the idea. A tween holds her phone in the selfie position, singing along to Selena Gomez's "Kill Em With Kindness" in her family living room. While it made its name with this type of short video, Musical.ly is now branching into new content, including acapella clips.

It's undeniably targeted towards young people. More than 60 percent of its users are between 13 and 20 years old, according to Fast Company. The app's user agreement stipulates that users must not be under 13, but confusingly adds that "by using the Service, you affirm that you are at least 18 years of age."

How strenuously that age limit is enforced is unclear. A quick browse revealed Australian profiles that clearly state the user ages as 10 and 12, among others. There were also numerous clips of girls in school uniform, school logo clearly visible. It would take minutes to track them down.

Musical.ly accounts are also incredibly public by default. User clips and location are viewable by anyone on the app, unless turned off in the settings.

On a section of its site titled "parents," the app advises parents to monitor their child's account for safety purposes. It encourages them to set the app to "private account" and also to ensure location information is hidden.

But does Musical.ly sexualise young people? It's hard to argue it goes further than other apps such as Instagram, which also encourage teens to present themselves in an appealing way to their peers and to the world.

Nevertheless, while users angle for likes on Instagram, Musical.ly makes that competitive edge explicit. Users compete to make it to the top of leaderboards or to be featured on the app. There's also a cottage industry of advice about how to be featured or win the "crown" symbol granted to top users.

In case you're unclear, the app's terms of service explicitly say it offers "the possibility of exposure and fame (is that Hollywood calling?)," as well as fun and creative expression.

In fact, top names on the leaderboard will be familiar to anyone who has dabbled in "Teen Internet." On Wednesday, controversial tween heartthrob Jacob Sartorius was number one with 10.2 million followers. Thirteen-years-old with floppy brown hair, he's the type that regularly tweets sweet nothings like "I keep having dreams about you 😊 " to his 896,000 Twitter followers.

Second on the leader board Wednesday was JoJo. Blonde, with braces and a big bow, she seems created in a lab for perfect tweens. In other words, it doesn't seem like a place for misfits.

Also, Musical.ly's top 20 doesn't appear to include any people of colour at the time of writing.

According to a Musical.ly spokesperson, the company prioritises the safety of its users. "We take appropriate measures to expeditiously remove offensive or inappropriate content from the musical.ly app," she added. "When a user flags content as inappropriate, it is removed from the platform within 15 minutes ... We are also implementing machine-learning technology to scan messages to block spam and inappropriate content."

To an "old" person, Musical.ly seems like a slightly terrifying means of self-expression. But yet, who among us did not sing into their hairbrush? Let's just turn on those privacy settings by default though, okay?

A Wenona spokesperson was not immediately able to comment when approached by Mashable Australia. Musical.ly has also been approached by Mashableregarding the apparent use of the app by people who are underage.