Big thanks to you quick response:)
I have not took my Pi to classroom, so i cannot verify your solution.
And i have one question for your solution:
For my understanding, piscreen connects RasPI with SPI interface, so, i think we should configure SPI as ON before we set piscreen as default monitor, what is your comments?
If i remember, one or two months ago, i have modifed /etc/modules to add some fbtft device configuration information when i followed the guideline 'PiScreen Driver installation instruction', i guess that the guideline maybe updated, would you please send me the previous version of 'PiScreen Driver installation instruction' if possible? because i want to configure other TFT LCD(not piscreen but notro fbtft support) for my PI, i think it is the best guideline i have read but i cannot find it from your website.