Regarding Low Register Flute

I'm writing a wind quintet piece where the flute part is frequently playing middle C or very close to it. While there's not necessarily a lot of competing with the full ensemble (at the same range), could an intermediate flute player (let's say high school level) play forte in a long phrase?

I think so. Actually i'm just about positive they could. If they have their hands on an intermediate level flute they could even go down to a low B.

so the answer to your question is yes, an intermediate level flute player should be able to comfortably play in a low range.

It won't be a strong forte. Flutes are naturally weak in their low register. While it may be possible to play at the desired dynamic you should bear in mind that the timbre will be very different than the rest of the range. It will be very hollow sounding.

Thanks to both of you, these were the exact answers I was looking for.