Shorts are not acceptable in public places.
Wearing ties or bows is not a problem.
T-shirts are acceptable.
Obeying Islamic rules including Hijab or Islamic dress-code is necessary in Iran. However these rules are not observed very strict, especially for tourists and foreigners. You must not worry about maintaining your hijab , since in times you have forgotten about it, the maximum penalty will be a request (usually in a kind way) to make it correct.
There are some minimum requirements for foreign women dress-code in public places :
1. Color: It's a completely false belief that wearing must be dark in Iran. There is no limitation in this respect and we recommend you make sure using light colors in summer.
2. Head: Hair should be covered. It does not mean you shall have a tight scarf around your head. Don't worry; it is very usual that some parts remain out of the cover. It's quite acceptable for women to allow whips of their hair to frame their face. Appropriate hats & caps can do this function as well as scarves. Scarf is the most common covering for head and is called "Roosari" in Farsi.
3. Body: Should be covered with loose clothes like man shirt, coat or manteau. Arms should not be bare.
4. Legs & feet: Legs should be covered down to ankles. Feet can be bare and you can wear sandals. Tight jeans are no problem.
*Tip 1: In summer and hot weather, you can cheat (!) and not wear shirt under your coat/manteau. Who understands?
*Tip 2: In winter time, apart from covering the head, the rest is the same as what you wear in your home country.
*Tip 3: You better enter Iran with a coat and a scarf and select your style by watching the Iranian women in the streets.
*Tip 4: Once a chador is needed, like in holy places, it will be given upon entrance.
*Tip 5: Following the dress-code is necessary form the time of on-boarding international flights. On Iranian Flights, it is usually requested when the plane enters the Iranian boundaries.