The ornamental design for an anti-theft alarm for book bags, as shown and described.
FIG. 1 is a perspective view of an anti-theft alarm for book bags showing
my new design, with the broken line showing for illustrative purposes only
and forming no part of the claimed design;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a bottom plan view thereof, with the top being a mirror image;
and,