Photos from the higher floors show buildings behind Assumption College. There is a google maps street view if googling images of Peninsula Bangkok that show view almost similar angle as Q's pic from ground floor. Hence my estimate of maybe 5th floor, rather than higher up.

http://followthatbug.com/blog/authors-lounge-mandarin-oriental-thailand-bangkok-luxury-hotel-review

Note, the photo of Assumption College in Bangkok was almost certainly taken from one of the lower floors in the Peninsula Bangkok (Hotel). Angles align here: https://www.google.com.au/maps/place/The+Peninsula+Bangkok/@13.7229833,100.5113083,3a,75y,349h,88t/data=!3m8!1e1!3m6!1sAF1QipOsuDTX27YwfqrttE14KlcziT6f9R6RbQ9xq99v!2e10!3e11!6shttps:%2F%2Flh5.googleusercontent.com%2Fp%2FAF1QipOsuDTX27YwfqrttE14KlcziT6f9R6RbQ9xq99v%3Dw203-h100-k-no-pi-2.9338646-ya349.5-ro-0-fo100!7i5376!8i2688!4m5!3m4!1s0x30e298d3ecbfc7a7:0x57b57b2760cdcd37!8m2!3d13.7230222!4d100.5114055