Continuity mistake: When young Picard sees his "father" Riker, he says to turn on the computer in schoolroom 8. In the scene showing the children gaining access to the computer, the terminal they are at shows classroom 7 across the header of the screen. (00:36:20 - 00:39:55)
Other mistake: When Ro Laren enters the Jeffries tube, she makes a sudden shift to her right (our left) as she begins crawling; she was trying to avoid the camera.
Answer: Yes. The doors to the holodecks can be locked when in use by anyone aboard the Enterprise so they couldn't be disturbed. However, high ranking officers like Captain Picard could override the doors as it's seen that overrides are in use even for the crews quarters. Even Barclay, when he's indulging in one of his fantasies could have the doors to the holodeck unlocked by an override code.