The film's screenplay makes it pretty clear that Neville and co. were speaking about blowing up the wooden bridge, the rearward entrance to Hogwarts Castle.
PROFESSOR McGONAGALL: Why don't you confer with Mr. Finnegan? As I recall, he has a particular proclivity for pyrotechnics.
Neville and Ginny glance back at Seamus.
SEAMUS: I can bring 'er down.
PROFESSOR McGONAGALL: That's the spirit. Now off you go.
In the following scene, they're doing precisely that, setting charges onto the wooden bridge. Note that this the screen direction is "continuous action" indicating that they've not been anywhere else since we last saw them, followed by "moments later".
235B - EXT. WOODEN BRIDGE - CONTINUOUS ACTION - NIGHT Further on,
small as ants from this vantage, Neville, Ginny, Seamus and half a
dozen DA members approach the WOODEN BRIDGE.
242 EXT. WOODEN BRIDGE - NIGHT (MOMENTS LATER)
Neville limps into our FIELD OF VIEW and peers outward, marveling at
the bombardment above.
SEAMUS: That's it. I want a charge on each and every joist.
Neville peers below. Down amongst the pilings, Seamus directs KATIE
BELL, CHO and NIGEL in placing magical "charges" (spells) on key
NEVILLE: You do know what you're doing, don't you, Seamus?
Seamus winks, gives a thumbs-up — he seems to be enjoying himself,
perhaps inordinately so. Neville exhales, shakes his head.