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In the snaps, Fox posed in a dusty blue sleeveless top with a keyhole opening at the chest, paired with matching slacks.
The Jennifer's Body star added a lavender tote purse and matching pumps, and complemented her look with an orange manicure and similar shades around her eyes.
"If I were the AP Literature teacher at Euphoria High," the mother of three captioned the post.
The centerpiece of the photos was Fox's lavish emerald and diamond engagement ring, which features pear-cut stones.
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Kelly, 31, had the ring custom-made by Stephen Webster before proposing to Fox earlier this year during a trip to Puerto Rico.
"She has been wanting to get engaged. The proposal was a surprise, though," a source previously told PEOPLE. "They have traveled to Puerto Rico before, so Megan just assumed it was another vacation."
According to the source, the actress "loved the proposal" and "thought it was beautiful and perfect."
"She is excited to get married," added the source.
In 2019, Fox split from actor Brian Austin Green, with whom she shares three sons — Journey River, 5, Bodhi Ransom, 7, and Noah Shannon, 9. Green, who is is also dad to 19-year-old son Kassius, confirmed his split from Fox after nearly 10 years of marriage in May 2020.
Kelly, meanwhile, has a 12-year-old daughter named Casie, whom he shares with ex Emma Cannon.