Trailing 20-16, Castleford snatched victory when Ben Wilkinson's try in the final minute was converted by captain Johnny Payne.
Both sides showed exemplary discipline in a thrilling derby.
There was excellent defending by both sides in the opening ten minutes, led by Castleford's Joel Penny and Airedale's Grant Richmond.
Castleford went ahead when Payne, Danny Peaker and Danny Smith paved the way for Kyle Williamson to score a try that Payne goaled.
With half-back Peaker in outstanding form, Castleford increased their lead when Siddons regathered his own neat chip to send in Payne for a try the scorer also converted.
Good work by Michael McCartan, Joel Penny, Peaker and Smith saw Castleford's Grant Susse score out wide before Airedale ralled and hit back with unconverted tries from Josh Hopps and Jack Smith.
Airedale now had the ascendancy and despite excellent defending from Castleford's James Underwood, Conor Condron and Luke Hobson, Airedale's Grant Barnfather burst through a tiring defence to score a try that Nick Fozzard goaled.
Airedale's outstanding pack man Lee then went over and Fozzard again added the extras to put them 20-16 up.
Airedale looked to be heading for victory until Scott Whiteley's powerful run set up the position for hooker Wilkinson to scoot over from acting half to level matters. Payne kicked the winning conversion.
Castleford, who were National Cup winners in year 8, have now finished their school rugby by being crowned the best team in Wakefield district.
The man of the match award was shared by Castleford scrum-half Peaker and second row Siddons who were both in inspirational form.
Siddons said: "It felt good to win our final game against Airedale. We have had lots of bruising encounters over the years and this was a great final to go out on. I'm proud that the lads never gave up, and we got our rewards."
Castleford: Tate, Underwood, Condron, Susse, Hobson, Smith, Peaker, Penny, Wilkinson, McCartan, Williamson, Siddons, Payne, Harriman, Brown, Whiteley, Wilson.
Airedale: Knowles, Asquith, Olbison, Berry, Firth, Winstanley, Fozzard, Richmond, Clark, Smith, Lee, winson, Barnfather, Hopps, Rothwell, Hancock, Paige, Wilby, Ellis.

Castleford High Year 7 ....24
Malet Lambert High (Hull) Year 7 ...22
A last-minute penalty goal by man of the match Tom Holmes gave Castleford victory in the Yorkshire Carnegie Champion Schools Cup final at the Chris Moyles Stadium, Featherstone.
Holmes gave Castleford the edge over their spirited Hull opponents who recovered from 22-12 to level the scores with four minutes remaining.
Malet Lambert took an early 6-0 lead but Castleford replied when quick passing led to Keiran Gill side-stepping past three defenders to score.
Malet Lambert got a second converted try to lead 12-4 but Castleford squared matters before half-time.
Liam Hands powered over from ten metres out before a quick play the ball from Castleford back rower Connor Hughes and excellent handling saw winger Tom Hargreaves cross at the corner.
Early in the second-half, Holmes converted his own 20-mtre interception try to put Castleford ahead for the first time.
They increased their lead to ten pointds when Ben Cringle crossed but Malet Lambert fought back to level at 22-22.
In the final minute, a Malet Lambert player was adjudged to have ripped the ball out in a tackle and Holmes kept his nerve to kick the resulting match-winning penalty in a game played in excellent spirit in which only three penalties were awarded.
Holmes was pushed close for the man of the match award by team-mate Callum Butler.

Castleford High Year 8 .. 26
Freeston Business College (Normanton) Year 8 ..27
Castleford were pipped in extra time in the Yorkshire Carnegie Champion Schools Cup final which was a repeat of last year's National Final when the two teams became the first-ever RL sides to play at the new Wembley.
Once again the sides produced another truly memorable final.
National Cup holders Freeston went ahead when Brad England's try was converted by Ryan Hampshire. Castleford replied when Luke Metcalfe scored out wide. Two minutes later, Jack Miller put in Mitchell Law for a try that Miller goaled to put Cas 10-6 up.
Freeston regained the lead with England's second try that Hampshire again goaled but Castleford stormed back to snatch a 22-12 advantage when Lewis Young and hard-working Liam Radecki both crossed and Miller added both conversions.
When Harper put in centre Chris Oakley to make it 26-12 with ten minutes remaining, Castleford looked to have the game won.
However, Freeston cut the gap with two tries from centre Liam Blockley before they squared matters in the last minute when Hampshire converted his own 80-metre try to take the game into extra time.
Hampshire kicked a drop goal in extra time to earn Freeston victory.
Radecki and Harper were outstanding for Castleford while England and man of the match Hampshire starred for Freeston.

Castleford High Year 9 ...40
Winifred Holtby High (Hull) Year 9 ...8
Castleford's big pack proved too strong for Winifred Holtby in Yorkshire Carnegie Champion Schools final at Grattan Stadium, Bradford.
Castleford's try were Jordan Gill (three), Shaun Pick (two), George Marchant, Connor Land and Bradley Ellis.
Stand-off Land landed three conversions and prop forward and man of the match Jack Cockerham kicked one.