Mercator, Belgian navy. An odd three masted barquentine rig with the foremast also fitted for square sails.
Look closer to home ... (and I can see no square rigging there, she looks a pure schooner) but another source I found says she is was not used as school ship but in a much more warlike role.
Ok, but that doesn't mean given the picture that she couldn't have just shipped the spars. That is very common on sailing vessels to not be running under full rig.
No she served as a military vessel from 1917 to 1919 and from 1941 to 1947, this shoild clearly point to her nationality though she served in different "forces" in the two wars.
Full hit , my book reported her as a schoolship, but it appears she served as a guard ship in WW1 as USS Atlantic II (SP-651) and in the coast guard in WW2 as USCGC Atlantic (WIX 271). Pick a nice one ...
Now this is nasty .... Assuming the white ensign is not a red herring ... looking at her stern I would rule out a WW2 corvette, though she's very similar in configuration and size looks about right. No visible AA and no visible electronics but the ship in the background and Carley Raft looks later than WW1 first guess the Southern Pride (she's the ship the Flowers were derived frombut can't find a picture of her).
The general configuration makes me think a WW1 minesweeper or ocean tug, passed along to some other navy between the wars.
Commissioned 30th June 1917, decomissioned 1919 then returned to owner. Sold in 1940 and comissioned into second navy 28 May 1940 and had a change of name. Then reclassified before decomissioning in 1944 and being declared surplus in 1945. Sold in 1946 and scrapped in 1957. Clue: photo shows her in her second world war comissioned state, minus her bowsprit.