As there are four distinct seasons in Finland, you will need very different clothes during your stay.
At the beginning of the autumn semester, you may need summer clothes as the temperature can be 20 degrees celsius. Later in the autumn (October - November) you need rain and windproof outdoor clothing. The temperature can be above or below zero during this period.
During the winter months (December - February) the temperature is usually below zero which means that you need warm clothes. The secret to keeping warm when it's cold outside is to wear layers of clothes: first an under-layer that draws perspiration away from the skin. Then another layer of warm clothing and, on top of this, a warm coat. You also need warm socks and shoes, gloves, a scarf and a hat.
In the early spring (March - May) you need rain and windproof outdoor clothing. The temperature can be above or below zero during this period. At the end of the spring semester, the weather gets warmer and you may need summer clothes, as the temperature can be 20 degrees celsius.
During the summer the weather in Finland can be warm, around 20 degrees. There are also periods during the summer when the weather can be colder, so be prepared to bring a jacket.