Ryan, 47, posted photos from the event, including one of him and his ex-wife, 46, beaming next to their younger child.
"Homeschool graduation," he captioned the images, noting, "I played principal" — and dressed the part in his own cap, gown and stole.
Ryan shouted out Reese for the "cap grab," as the Legally Blonde star retrieved Deacon's orange graduation cap from the pool after the teen's celebratory toss.
He also showcased the 18-year-old's diploma, which was was a free internet printable, fitting with the laidback vibe of the event. The document was signed by his star parents.
The event was followed by dinner out, which Ryan shared a glimpse of in his Instagram stories.
Deacon appreciated his dad's posts — and commitment to his role in the DIY graduation. "Best dad ever," the new grad wrote in the comments.
Also in the comments, fans were happy to see the exes — and Cruel Intention co-stars — together again supporting their son. There was also a comment from actor Justin Long, who wrote: "All three of you could be recent high school graduates!," adding the hashtags "Vampire Family Vibes" and "Fampire."
Deacon recently attended the prom, which Reese shared a snapshot of, writing, "Love this guy."
Deacon is already a budding EDM star, releasing several singles already and frequently sharing new music on TikTok. Reese got a shout out ("Sorry mom") in this one:
Reese and Ryan, who also share daughter Ava, 22, were married for seven years before splitting in 2006. She has been remarried to talent agent Jim Toth since 2011 and they are parents to son Tennessee, 9.
Ryan also has a 10-year-old daughter, Kai, with ex-girlfriend Alexis Knapp.