A long story trying to cut it short but don't want to drip feed.
DD(14) and her friend 'Kate' (and their sibling) skipped school one day last week...and Kate's dad has covered for them ...and I'm furious...
Bad weather meant the school buses were very, very late but DD managed to get on one. Then she sent me a series of texts telling me there was no point going in, there was no-one on the bus (lots had given up but turns out a lot got lifts in), someone's dad was going to pick them up and take them home in case the weather got worse ...etc, etc. I told her the forecast was for the weather to improve -she was on the bus she should go into school.
A couple of hours later I got a text from the school asking where she was. I phoned them and was told it might be a mistake, she might be in then got another saying she definitely wasn't in and a friend had said they thought she was in a nearby shop. 
The school said she was classed as 'missing' so I told them everything she had said in her texts inc 'Kate' and sibling (who live near us) had been on the bus.
(DD wasn't replying to texts or answering her phone at this point)
And I told them to throw the book at her, agreed to a detention for her this week - she should and could have been in school.
They called again - another friend had said she was at Kate's house
(whose parents are both usually out in the day).
I was on my way round there when I got several of the same text saying she thought she should go back and go to Kate's to do some revision (she says it must have been delayed and on her phone it does say it was sent before 10am but I'm dubious...seems too convenient when her friends in school would have just messaged them that the school knew they were skiving ...and I had got the texts/calls from the school etc no problem)
Then I got a call from DD she told me they were scared to go into school so late
so had got straight on a bus back. And Kate's dad had seen them and said it was fine. And Kate (and sibling) were scared they were now in trouble too.
It was just gone 1pm by now and I told her to get on the next bus into school... (it is over 10 miles away, takes the best part of an hour to get there but it was the principle). Then she phoned me and said Kate's dad had spoken to the school and it was all ok - and they had agreed there was no point going in then.
I was a bit pissed off and phoned the dad and he said I knew what they were like - they were just too nervous to go in
(like I said all the buses were late, there would have a group of them from that bus going in, they would have known they wouldn't be in trouble). He had seen them coming home and not to worry
the school were fine with everything
.
Then in a series of texts with DD she said they had messed up, thought it was a bit exciting, wanted to be naughty for once (they are generally very good and well behaved).
I saw Kate's mother that evening and she said she really felt for them - how scared they must have been not to go in after going all the way there etc - I was
and told her about what DD had said but she didn't want to know.
This weekend DD let slip that they knew about the forecast, had talked about it the day before, thought they probably wouldn't get to school that day - had more or less planned it...
I told DD as far as I was concerned she would get the detention and I wouldn't cover for her. She said but Kate would be in trouble too.
I said I'd let the school decide....
Then yesterday asking when her detention was, DD tells me Kate's dad has written to the school, said he told them to come home, told DD to come back with them as they couldn't contact me
and thought I'd want her safe 
- basically as DD (gleefully) told me - he has lied for them...
I am friendly with the parents - and actually Kate is generally a good influence on DD. I know it is tricky as either they were both 'innocent' or both 'guilty' and we have different parenting approaches to this. (They are more laid back about attendance anyway - Kate and sibling have a day off sick every other week.)
And they seem to want to believe the 'too scared' story - I think they are being hoodwinked -not an easy thing to tell another parent.
I think it an important life lesson - you do something naughty/stupid and get caught, you take the consequences. (in this case one detention is pretty minor)
I think I just feel angry my feelings on this have been ignored. (Then I guess though they do have two DCs involved.)
Also if the school phone me what do I say? That the dad is lying?
So AIBU to be really annoyed?
(I have to say I know it is only one day and I might be overreacting -but I started school refusing at DD's age - I don't want her to realise how easy it is to do that etc - its why I am so hot on attendance...)