The backline of Eddie Nolan, Michael Nelson, Paul Reid and trialist left-back Gavin Gunning were repeatedly exposed as the Iron conceded all five goals in the opening half.
Although the Championship visitors played well, they were able to rattle in their goals without anything resembling a decent tackle to stop them.
"Our first-half was pretty poor. Barnsley gave us a footballing lesson. The goals were shocking," said Knill.
"We were better in the second half. Overall though, it was pretty poor.
I hope it provokes a reaction at Wycombe next Saturday. I expect better from everybody."
There were a few small positives for supporters: the second-half was more balanced, and as the game entered the final stages the Iron created several chances.
Garry Thompson and Michael O'Connor also came off the bench - contributing largely to the late improvement - to give hope they may be able to play a part in next week's League One opener at Wycombe.
"There were some good points to take from it. Jimmy Ryan was by far the best player and looked creative. He was superb throughout the game," added Knill.
Knill also confirmed that the club had received an approach for Andy Hughes, and he was left out of the squad as a result.