Mike would have to be really subtle about using 'Observe' in class, since it was a voice activated Skill and it wasn't exactly something you could whisper inaudibly. So he would not be able to use it as quickly. Luckily most people were involved in their own conversations. Unfortunately Mike's observe spree would end a bit early, as the teacher walked into the room to start class. After the greeting and roll call, which Mike would have had to remain quiet for, class and the school day itself would start. Their next break would be lunch, and if Mike checked, the Skill had barely gained any XP at all from those usages of it on all of the students, mainly because he had used it on them before the first day he was here, and the only XP he did get was probably from whatever new information that appeared, which wasn't all that much. 

Ren, even during class, didn't take notes. He just kept reading that same book the entire time. Even during lunch, as people went to eat with their friends or talk, Ren just did nothing except read and eat from his own bento in solitude. A few students, who were interested in the new kid and wanted to be friends, would ask Mike if he wanted to eat lunch with them as Mike was friendly and sociable enough to actually get such offers. Mike could either try to make new friends with these three random students, or do something else with his lunch period, which wasn't that long for anything big, but for just visiting or conversations it would do well enough. 

After all, despite having this power, it was important to try and live normal lives as students as well, something which Keiichi had stressed was very important several times, and was one of the main reasons why he wanted Ren to go to school instead of skip.