Actually, it may be easier to get A's in that type of environment as the parents expect it and more of the students earn it. There is an extremely wide disparity in the "level" of academics within the the U.S., within each state, within each county, within each district and even within each school. However, it often seems that more higher level grades (A's and B's) are awarded in the more competitive environments, rather it is GT versus standard classes or top school versus struggling school.
Another consideration is that there are probably plenty of gifted students at that type of international school so your DDs will be in good company. I would also caution against certain assumptions that may cause your DDs unnecessary stress. There have been countless times when I have let other parents assume that my kids actually has to work hard in their classes (or a competition, etc.) when that wasn't true.
