Student or Learner
I mean... they were just kids. I mean, it was hysterical to watch their parents, alright...asking these kids like they didn't know what they were doing. Here they were, caught in cold blood. Ok? At this party. And the parents are still maintaning, with composure, "What's that pipe doing in your hand?" But you know something? I'll say this, it forced the kids to answer honestly
What does "I'll say this" mean in this context?
You couldn't just f** Jane. You had to have Mark as well. I think we can call this an evening.
What does it mean?
Thanks for you all helping.
***Not a teacher***
In my opinion, "I'll say this" in this context is introducing a view held by the speaker, which is contrary to his/her general flow, or the expectation of the hearers.
I'll try and demonstrate what I mean by an example:
"The new car is awful, it's clunky, looks ugly and drives like a tank - but I'll say this, it is a lot cheaper to run"
The second one is a bit more difficult without a little more context.
To 'call it a <something>' means that is enough for now. So "Let's call it a day" means we should stop what we are doing now until another day. Similarly, though less commonly, you could say "We can call it an evening", "We can call it a night" etc - meaning lets stop what we are doing now and give up. However, I wonder if in the context given above whether something else is meant.
The fact that it says "I think we can call this an evening" rather than "I think we can call it an evening" suggests that this has a different meaning. Perhaps it means something more along the lines of:
"I think we can safely say this is an evening we are going to look back on and remember" - in other words "this is an evening" means it won't just be forgotten, but that it will be remembered as an occasion because of the events with Jane and Mark.
In both cases, I think it is idiomatic and would only be used in speech, and judging by the context, carries elements of urban/street talk.