WalkingthePlank:
Have absolutely run into this with DD2 - both at old primary and new primary she used to miss maths for violin lessons. Really hated that as a parent.
Our solution was to do more at home - and basically use what homework was coming home from old primary (and regular homework from new primary) to gauge whether DD2 understood or found a concept tricky. If it was tricky we'd try to do more/ work on technique a bit more at home. When we moved schools in Y4 DD2s new school would always apprise her of what the class had been working on and let her take any unfinished worksheets from maths home. Occasionally I'd have a little note (Post-it or on worksheet) telling me that they were working on X - and could we practice a bit at home as DD2 only had a short time on the topic due to music lesson.
Ultimately - as DD2 has moved up years (now Y5) the timing has changed - in fact she's now first thing in the morning - at worse missing registration.
I certain query why your child is getting English only worksheets for MFL - that shows lack of cooperation (certainly the class resources - whether they pre-exist or are designed by the teacher his-/herself are likely to be digital - so as a parent you simply need the web address or the teacher to e-mail the powerpoint).
after 4 p.m. Linguascope: www.linguascope.com/ - is free - and there is a lot of common vocabulary - games, etc... resources there - so simply knowing you were covering animals is spanish is enough - and you can follow links to 'animals' and learn more there. Not ideal - but will help.
BBC has a free website for primary languages here: www.bbc.co.uk/schools/primarylanguages/ - covering Spanish/ French/ Mandarin.
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If you feel up to going to battle my advice is raise this with the HT. See what comes back.
If no joy - Then if the music is taught peripatetically and funded through county/ city council - raise it with your LEA. Because this is a school willfully impairing your child's education and forcing you to choose between language or music. [Frankly LEAs you should be looking into this - because this is a pretty common complaint from parents with pupils learning an instrument in school through individual or small-group lessons].
At same time - raise this generally with Governors - not as a specific case (although you have your particulars as an example) but as a general issue vis a vis all pupils learning an instrument. The school should be making adequate provision to 'catch up' a pupil missing part of a class for an instrument lesson. The teacher/ TA/ fellow student should be apprising them of what is going on and bringing them up to speed. Any missed worksheets/ resources in class - could be sent home (paper copy or via e-mail).
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Finally schools/ teachers - given you are fining parents for missing school - I think you we're moving into a quid pro quo situation whereby schools will be required to demonstrate they value unavoidable loss of lesson time (field trips/ special assemblies/ closure for poor weather/ etc...). You may have had no choice on the peripatetic music teaching day/ hours - but you do have control of planning to accommodate this - which may include sending unfinished worksheets home with pupils having lessons/ sending webpage links or e-mailing digital resources to parents/ one-to-one time with returning pupils to get them up to speed and rejoin main class work. Obvious solutions include planning lessons during breaks/ assemblies/ before or after school (which are low educational value) and/or planning that pupils with obvious weaknesses in a core subject (e.g. reading/ writing/ maths) are taught where possible at alternate times from these key 'learning times' during school day.
Certainly at our first primary - support of peripatetic music teaching from the school was limited - often lessons were in corridors because the usual room was needed for some other activity (including storing sets for Christmas play). They trumpeted the arrival of 'whole class' music lessons - guitar - which ended up with Year 4 strumming the same note (just the one) for an entire assembly. (Not exactly what I'd call 'learning an instrument'). I think too many schools keep music lessons because it is attractive to parents (ticks the box) - but don't really support music lessons or get the unquantifiable benefits for pupils learning an instrument - improved memory, concentration, etc....
As ever - I seriously doubt OFSTED inspects this element of provision and as it doesn't feature on 'league tables' some schools may feel why put much effort in?