Ambling I can't imagine the DC still being at school now; they were ferienreif a month or more ago and it's week 3 of the holidays here now - mind you, I think the fact that you know the holidays are about to start always makes you ferienreif. But they only go out on trips in the last week, so it's hardly taxing.
NurNoch I don't buy things like paintbrushes and pencils at Hofer as I'm not sure about the quality, but I stock up on exercise books and covers, Schnellhefter, Klarsichthüllen, etc. I have Schnellhefter and exercise book covers in a huge array of colours so am rarely caught out when it comes to those, but do need to go to the stationery shop for A5 and/or unlined exercise books, or anything more specialised than lined, squared or lined with margin A4 or Quart. It still really cuts down the amount I need to buy in September, though, and also during the school year - the DDs always seem to be running out of exercise books and need a replacement by the next day, so it saves a special trip to the stationery shop.
Re school bags for secondary, DD2 has this bag from Jako-o and DD1 has a fairly similar bag from the stationery shop, which she'll still be using in her final year of middle school (8. Klasse in Germany, I think).
And yes, WhatsApp use is very widespread here even among children in their first year of secondary school. I know two mothers who were quite against their DC having smartphones but ended up buying them for their DC as they felt they were being left out - the form teacher had set up a WhatsApp group for the class in the first week of school (both DDs' classes have WhatsApp groups too, but set up by the children so not official). WhatsApp has been great for DD2 as her closest friends have all gone to different schools so she uses it to keep in touch with them; she has a PAYG phone, so it's cheaper than phoning (unless she uses Viber) or texting.
DD2 enjoyed the scout camp, especially the hike - they were divided into groups of four or five, given a map and had to walk 12 km to a place where they stayed overnight (they met the scout leaders there) before hiking back to the campsite again the next day. Apparently the other group got lost and had to phone to be picked up (they were given a very basic emergency phone) as they wouldn't have arrived until 2 am otherwise! DS left for his scout camp at the weekend, but we'll have to pick him up on Thursday as we're off on holiday on Friday so he won't be able to stay the whole week.