what makes you say that Lexington and Concord showed the British at their highest point? 

what makes you say that Lexington and Concord showed the British at their highest point?  While these two battles were not decisive victories for the colonial militia, I don’t think they could be counted as victories for the British either.  Lexington started the war with the “shot heard around the world”, but Concord showed the British retreat back to Boston due to the militias quick ambushes along the march.  Boston was eventually taken by the militia and Continental Army, as they were able to blockage the routes Boston and trap the British regulars in a siege for the city.