A bit about honorifics in Haru vol. 10&11... In 11, ch. 1 there is a lot of Onozuka-Miyasaka-Iwaki-Katou action, and the use of honorifics says quite a lot about their relationships at this point.
First of all, this made me rethink the relationship between Iwaki and Katou. We have assumed that it is Iwaki who holds back, mainly based on the fact that Katou and he have had a discussion about given names, and when to use them. But if we look at how Iwaki and Katou address each other and how they speak about each other to other people, it is clear that Iwaki's relationship with Katou is more... intimate than we think - he is not holding back. No, it is actually Katou who might be the one who (inadvertently, perhaps) who have set the up barrier which prevents that the family names are put away. It is Katou, after all, who tells Iwaki that he'll reward him by calling him Kyousuke at the Golden anniversary whereas Iwaki wants call Katou Yousuke already at their Silver ann. :D

Here is why: Iwaki always - always - adresses Katou with no honorific. He speaks of him in the same way, thus signaling that their relationship is a deep and intimate one. Katou, on the other hand always uses the respectful -san, both when he speaks with Iwaki, and when he speaks of him. -San is of course a vastly more intimate address than "Mr" - which can in no way be compared to -san. This sets up an invisible barrier - it is one of respect and care, but a barrier nevertheless.

Now, on to Miyasaka and Onozuka. It is obvious that the two fools young men are casual with honorifics. They call each other -kun, and Onozuka makes a small distinction (maybe because he's the brain) and calls Miyasaka Miya-chan as often as -kun. -Chan is of course a loving address, between very good friends, more intimate than -kun. It happens often, especially if they're angry at each other, that the honorifics are dropped. The same goes for the their respectful language when it comes to Katou. Miyasaka and Onozuka also speaks of Katou without the -kun, just as Katou addresses the both of them with or without the -kun added. Katou, on the other hand, is angry when he thinks Onozuka dares call Iwaki Nii-san (in ch 2/36).

It should be noted that honorifics are never used when a worker from one company speaks of a colleague to others. This is a part of the humble language (keigo): one never raises up one's own company by using honorifics added to co-workers' names. I don't think this has anything to do with the three friends here, but it is worth remembering.

On an entirely different note... Mochimune... his name seems to be written with two non-touyou kanji, which means that they are not on the government-approved list of approximately 2,000 kanji that one needs to know to read modern Japanese (I suck at kanji, so I might be wrong here). However, the name Mochimune *is* an old high-ranking bushi (daimyo, shogun, shogunate administrators-all warrior nobles) name from the time that only 20% of the Japanese (namely the noble and the bushi - the samurai) were allowed family names. Might explain his immense arrogance. *loves on him*