Sports Day
You may write about your child's reactions and your own impressions after a Sports Day event. You may of course speak to the teachers directly instead of writing in the renrakucho. The following model contains two "Thanking the School" sections, but you may choose to include just one of them. You may also choose to make just a single comment regarding your impressions and/or your child's attitude.
Model
Conversational flow and useful expressions
Thanking the school 1
Impressions 1
Child's attitude, etc.
Impressions 2
Thanking the school 2
Parent's name
Supplement
「お疲れ様でした」(otsukaresama deshita) is used to thank teachers for everything they have done, and also to express appreciation for the energy and effort they have expended. 「ご苦労様でした」(gokurosama deshita) has a similar meaning but may at times sound condescending and even rude, so it is probably best to avoid using it.