I don't have a tattoo, but I worked with many people who do and I don't think they had any problems. Going barefoot is the norm in Thailand so your foot will be exposed in class unless you wear socks. Not sure what the rules are on nose piercings though. Some of my colleagues had them but I can't remember if they removed them for work. Err on the side of caution and ask for the dress code in advance. Best of luck!