The correct verb form is "remain"... AND the circle is still singular. ;) You've come across "present simple subjunctive". It turns up much more often than you think. Basically, the verb doesn't change from its base form, even if the subject is singular he/she/it. This form comes after certain verbs plus "that" ("I demand that she apologise!"), and certain phrases, such as "it is essential that". When the verb is in present subjunctive form - ie. unchanged - we understand the action as an idea and not existing in reality. Using "remains" is actually illogical. That would mean trying to use a "real" action when only an unreal action is correct. Here's a neat little mini-tutorial: http://www.englishpage.com/minitutorials/subjunctive.html

It's not being used as a collective noun. The verb is in the subjunctive mood. http://www.englishclub.com/grammar/verbs-subjunctive.htm

Ah, Jmat, I am a little disappointed now. Almost every time I find an error in my English grammar, people tell me not to worry about it, since even native speakers do it wrong. Now I was happy I finally found something and you tell me.... ;-) ...But thanks for the information!!! :-)

The subjunctive mood is largely ignored in casual conversation. You'd likely hear "...that the circle remains..." more often than the technically correct version, and I doubt many native speakers would even find the former incorrect at all. Don't be surprised if you don't hear the subjunctive when it should be used.