No-one was injured and it is still unclear how or why the bus was on the Circuit of the Americas in Texas during the F4's second race, although school buses are used collect marshals at the end of the day.
Leitch (21) was just thankful that the call came over his radio that a safety car had been deployed and he had buttoned off from race pace. He saw the 30-seater bus in the distance and initially thought it was on the other side of the fence.
''Then I had a wee panic moment because it's actually on the track.
''Luckily we had slowed down. It was the weirdest thing I've seen as a race driver in my whole career,'' he said.
Leitch, who works at Highlands Motorsport Park, returned to Cromwell this week after a seven-week stint in the United States to some good news. His Kiwi Motorsport team phoned to say it had located and remedied several issues that had been hampering his F4's performance recently.
It was a relief for Leitch who had found last weekend's Texas round tough due to having a car that was down on speed.
''We were hoping to be very competitive here, but the other teams had speed we couldn't claw back,'' he said.
He qualified only 13th of 33 and was vulnerable to being dragged into incidents involving other racers. A penalty from the previous round dropped him further, to position 22 for the race one grid. Leitch put in a determined drive and clawed his way through to 16th at the finish.
On the grid one place higher for race two, his race was disrupted by the presence of the bus but once it had been escorted to the infield, he was able to climb back to ninth, banking valuable points for his championship campaign.
He finished 11th in the third and final race.
Leitch said a speed trap at the end of a straight was recording the pace of the F4 cars and his was the fourth-slowest out of 34 cars, making it hard work to progress through the field.
He is ninth overall heading into the final round, which returns to the Circuit of the Americas next month as a support category for Formula One's US Grand Prix. Crowd numbers are expected to swell to up to 250,000 which will ''definitely by far'' be the biggest crowd he has raced in front of.
