Class is in session for Amber Portwood and Gary Shirley's daughter Leah. 

The 14-year-old recently started a new school year, and her dad documented the major milestone. 

Taking to Instagram Aug. 3, Gary posted pictures of Leah in her first-day outfit—which consisted of a green top, blue jeans and white sneakers—as she carried her school bags and thermos while smiling next to his 8-year-old daughter Emilee, who he shares with his wife Kristina Shirley. (The couple married in 2015 years after he and Amber split.)

"Just two of the most important people on this planet," Gary captioned the snapshots of the kids. "Today was their first day of school! I love these girls very much. I'm very honored to be their father:)." 

And fans couldn't believe Leah is already a teenager. "Omg where has time gone," one commenter wrote, "she's grown." Added another, "Beautiful time flies by so fast." 

Indeed, it was in 2009 when fans first met Amber and Gary on 16 and Pregnant. The duo then continued to document their experiences, including Amber's legal battles and mental health journey, on Teen Mom and its spinoff series.

The show has also given insight into how Amber and Gary co-parent and how her bond with Leah has evolved over the years. While the 33-year-old has previously acknowledged they had a "strained" mother-daughter relationship, they've since both indicated that things have gotten better.

"I want us to be friendly together and not have any issues or anything," Leah said on a September episode of Teen Mom: The Next Chapter after Amber attended her 13th birthday celebration. "I don't want to have any tension or be awkward with her or anything."

As she put it, "We're taking a big step into having something."

In addition to Leah, Amber is mom to 5-year-old James, whom she welcomed with ex Andrew Glennon. However, last year, she lost custody of her son to Andrew and he was given the right to relocate the child to California, according to court documents obtained by The Sun.

Afterwards, Leah reached out to Amber to send her a loving message, and the MTV star expressed how much she appreciated it.

"Having Leah in my corner right now has been beautiful," Amber said on an October episode of Teen Mom: The Next Chapter. "When she sends me these messages, I feel like my world is just bursting with joy. She just makes me feel amazing."