StudentType.php

<?php

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;

class StudentType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        // ...

        $builder->add('score', 'integer', array(
            'value' => ...
        ));
    }

    public function getName()
    {
        return 'student';
    }
}

TaskType.php

<?php

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;

class TaskType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        // ...

        $builder->add('activities', 'collection', array(
            'type' => new StudentType(),
        ));
    }

    public function getName()
    {
        return 'task';
    }
}